Update documentation style & fix room version heading order (#3683)

* Update documentation style & fix room version heading order

Also add a missing signing key section to v6.

This additionally contains various edits to the fragments to have them make a little bit more sense in context.

Finally, this updates various aspects of the documentation style which haven't previously been considered - they're added here considering we're in the area.

* changelog

* enhanced changelogs

* Minor wording adjustments

* Apply suggestions from code review

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

content/rooms/fragments/v3-handling-redactions.md

@@ -5,7 +5,7 @@ toc_hide: true
 {{% added-in this=true %}} In room versions 1 and 2, redactions were
 explicitly part of the [authorization rules](/rooms/v1/#authorization-rules)
 under Rule 11. As of room version 3, these conditions no longer exist as
-represented by the above rules.
+represented by [this version's authorization rules](#authorization-rules).
 
 While redactions are always accepted by the authorization rules for
 events, they should not be sent to clients until both the redaction

content/rooms/fragments/v4-event-ids.md

@@ -10,5 +10,5 @@ Base64](/appendices#unpadded-base64) which replaces the 62nd and
 matches [RFC4648's definition of URL-safe
 base64](https://tools.ietf.org/html/rfc4648#section-5).
 
-Event IDs are still prefixed with `$` and may result in looking like
+Event IDs are still prefixed with `$` and might result in looking like
 `$Rqnc-F-dvnEYJTyHq_iKxU2bZ1CI92-kuZq3a5lr5Zg`.