* rename protocol ID to network capability

* guide: `ProtocolId` -> `NetworkCapability`

* guide: remove `RegisterEventProducer`

* capabilities and expand on underlying network assumptions

* guide: create network.md types file

* guide: network bridge is aware of network messages

* revert changes in code

* Update roadmap/implementers-guide/src/SUMMARY.md

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

* remove references to NetworkCapability

* Update roadmap/implementers-guide/src/types/network.md

Co-authored-by: Sergei Shulepov <sergei@parity.io>

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>
Co-authored-by: Sergei Shulepov <sergei@parity.io>

* Do a small write-up on collation-generation

* preamble to collator protocol

* notes on protocol

* collation-generation: point to collator protocol

* fix missing bracket

* expand on collator protocol wire protocol

* add a couple more sentences

* expand on requests some more

* go higher level

* network bridge: note peerset

* note peer-set = validation for protocols

* add `ConnectToValidators` message

* use ConnectToValidators in collator protocol

* typo

* remove references to sentry nodes

* rename implementor's guide to implementer's guide

* fix typos in more places

* rejig candidate types a bit

* adjust doc

* describe basic PoV distribution

* replace some straggling html links

* address grumbles from review and fix links

* Add mdbook-linkcheck

* Convert todo to not a link

* Fix external glossary link

* Fix various broken links

* Mass replace: s/.html/.md/

* My sloppy attempt to add a gitlab CI check

* adjust to relative links

* additional adjustment to relative link

* link directly to file

* link directly to file

* 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