High-Level Documentation (#565)

* High level docs - start.

* Clean up README

* Start adding details to high level docs

* More docs on the header sync pallet

* Testing scenarios document.

* Add some scenarios.

* Add multi-sig scenario.

* Start writing about message dispatch pallet

* Move content from old README into PoA specific doc

* Apply suggestions from code review

Co-authored-by: Andreas Doerr <adoerr@users.noreply.github.com>

* GRANDPA for consistency.

* Describe scenario steps.

* WiP

* Add notes about block production and forks

* Update.

* Add sequence diagram for Millau to Rialto transfer

* Clean up header sync pallet overview

* Remove leftover example code

* Clean up testing scenarios and amend sequence diagram.

* Linking docs.

* Add some more docs.

* Do a bit of cleanup on the high-level docs

* Clean up the testing scenario

* Fix typos in flow charts

* Fix small typo

* Fix indentation of Rust block

* Another attempt at rendering block correctly

* TIL about lazy list numbering in Markdown

* Add list numbers across sections

* Start counting from correct number

* Update README to use correct path to local scripts

* Wrap ASCII art in code block

Co-authored-by: Tomasz Drwięga <tomasz@parity.io>
Co-authored-by: Tomasz Drwięga <tomusdrw@users.noreply.github.com>
Co-authored-by: Andreas Doerr <adoerr@users.noreply.github.com>