* move old implementers' guide, add skeleton of new

* Split the old implementers' guide into the new one's sections

This is mostly a straightforward copying operation, moving the
appropriate sections from the old guide to the new. However, there
are certain differences between the old text and the new:

- removed horizontal rules between the sections
- promoted headers appropriately within each section
- deleted certain sections which were in the old guide's ToC but
  which were not actually present in the old guide.
- added Peer Set Manager to the new ToC

* remove description headers

It is redundant and unnecessary. Descriptions fall directly under the
top-level header for any given section.

* add stub description of the backing module

* add stub description for the availability module

* add stub description for collators

* add stub description for validity

* add stub description for utility

* highlight TODO and REVIEW comments

* add guide readme describing how to use mdbook

* fix markdownlint lints

* re-title parachains overview

* internal linking for types

* module and subsystem internal links

* .gitignore should have a trailing newline

* node does not have modules, just subsystems

* define subsystems skeleton

* Create a blank section for each subsystem in the initial batch

* write out basics for most subsystems

* define network bridge subsystem

* integrate network bridge into network systems

* Apply suggestions from code review

some nits

Co-authored-by: Max Inden <[email protected]>

* clean up wording

Co-authored-by: Max Inden <[email protected]>

* define subsystems skeleton

* Create a blank section for each subsystem in the initial batch

* write out basics for most subsystems

* Update roadmap/implementors-guide/guide.md

clean up language

Co-authored-by: Peter Goodspeed-Niklaus <[email protected]>

* ToC indentation

* clean up wording

* renaming block authorship (provisioning) to Provisioner

Co-authored-by: Peter Goodspeed-Niklaus <[email protected]>

* expand subsystems descriptions independent of subsystems overview

The subsystems overview in https://github.com/paritytech/polkadot/pull/1161
is a very useful reference for what subsystems need to talk to each other,
when, and why; it helps us design the actual messages.

However, administratively, it belongs in a different PR. This commit
brings in all the changes made there so far as a base for an independent
PR.

* Reorder subsystem descriptions, add some messages

Update ordering of subsystem descriptions to rough order of use,
mirroring the order in the overview document.

Added some message types. Added OverseerSignal variants to several
types, such that each subsystem only needs to listen for a single
type.

* add some more message types, Statement Distribution overview

* add more detail on Statement Distribution, Misbehavior Arbitration

* intentionally punt MA details for a future PR

* reduce duplication from overseer signal handling

* reword for clarity

* clarify: other modules and subsystems also get to talk to the network

* finish current work on candidate selection

* update candidate backing subsystem description according to current thought

* update mechanism for candidate backing to report collator misbehavior to candidate selection

* sketch out the peer receipt state machine

* Fix typo in roadmap/implementors-guide/guide.md

Co-authored-by: Robert Habermeier <[email protected]>

* Don't specify 'peer validators', as messages from non-validator peers are ignored regardless

* clarify instancing of peer receipt state machine

* add section on peer knowledge tracking

* fix typo in roadmap/implementors-guide/guide.md

Co-authored-by: Max Inden <[email protected]>

Co-authored-by: Robert Habermeier <[email protected]>
Co-authored-by: Max Inden <[email protected]>

* clarify exception to communication rule for responses

* add note about outside world

* write up my understanding of local and remote disputes

* clean up secondary checker language

* amend to match Jeff's notes

* migrate subsystem expansion PR from w3f/research

As the file at this new location included changes not present in
https://github.com/w3f/research/pull/87, this is effectively a
rebase, applied manually. I believe that I have successfully retained
all of, and only, the intended changes.

* add section on collators

* note why the overseer is the terminal message point for the validation subsystem

* add detail about how the validitiy system gets PoV blocks

* rename to Validity Subsystems Overview

* get rid of changes to the implementor's guide

I think it makes most sense to track and review the subsystems
overview in a different PR than the one which adds content to
the implementor's guide.

* punt misbehavior arbitration details to the future

* empty commit to rerun CI

* note pruning of orphan blocks in the overseer description

* remove note about finalization events being maybe not needed

* file structure and initializer skeleton

* ensure session changes happen before initialization

* add a couple tests for initializer flow

* integrate with session handling

* configuration update logic

* configuration methods

* move test mock to its own module

* integrate configuration into initializer

* add note about initialization order

* integrate configuration module into mock

* add some tests for config module

* paras module storage

* implement paras session change operation

* amend past code pruning to fully cover acceptance period

* update guide again

* do pruning of historical validation code

* add weight to initialization

* integrate into mock & leave notes for next session

* clean up un-ended sentence

* alter test to account for double index in past code meta

* port over code-at logic test

* clarify checking for conflicting code upgrades

* add genesis for paras, include in mock, ensure incoming paras are processed

* note on return value of `validation_code_at`

* implement paras routines from implementor's guide

* bring over some existing tests and begin porting

* port over code upgrade tests

* test parachain registration

* test code_at with intermediate block

* fix warnings

* clean up docs and extract to separate struct

* adjust implementor's guide to include replacementtimes

* kill stray println

* rename expected_at to applied_after

* rewrite ParaPastCodeMeta to avoid reversal

* clarify and test interface of validation_code_at

* make FutureCode optional

* rename do_old_code_pruning

* add comment on Option<()> to answer FAQ

* address some more grumbles

* remove old runtime requirements section

* reorganize guide and improve ToC

* remove redundant resize instruction from guide

* amend past code pruning to fully cover acceptance period

* update guide again

* note on return value of `validation_code_at`

* rename execution cores to availability cores

* clarify terminology

* add ReplacementTimes to the guide

* fully decouple inclusion and scheduler

* clarify parathread scheduling substantially

* clarify validator group selection

* include implementor's guide in this repo

* most updated guide