* Copy spec PR near-verbatim from past PR
With light review being addressed.
https://github.com/matrix-org/matrix-doc/pull/3168
* Alter for modern day
* Add changelog
* specify that we're using the grammar
* Cut/paste room version spec to its own page
* Move grammar to bottom + add feature matrix
The version grammar is not as interesting as the actual room versions, so this moves that whole section to the bottom.
* Fix all links to room versions
The Pagination section in the C-S API was, basically, full of rubbish. I think that anything of any value it contained was repeated either directly on the API definitions or in the text specific to syncing at https://spec.matrix.org/unstable/client-server-api/#syncing.
The conventions I've added to the Appendices are based on the discussions in #1898. They are there because I don't want to have to go through it all again next time we add a paginated API.
Fixes: #1898Fixes: #2268
