Minor cleanup on recently ported doc pages (#11466)

* move wiki pages to synapse/docs and add a few titles where necessary

* update SUMMARY.md with added pages

* add changelog

* move incorrectly located newsfragment

* update changelog number

* snake case added files and update summary.md accordingly

* update issue/pr links

* update relative links to docs

* update changelog to indicate that we moved wiki pages to the docs and state reasoning

* requested changes to admin_faq.md

* requested changes to database_maintenance_tools.md

* requested changes to understanding_synapse_through_graphana_graphs.md

* add changelog

* fix leftover merge errata

* fix unwanted changes from merge

* use two spaces between entries

* outdent code blocks