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>