Incorperate Levans' guide into official documentation

[aaronraimist]

The author of https://levans.fr/shrink-synapse-database.html said the blog post may disappear in the next few months https://matrix.to/#/!ehXvUhWNASUkSLvAGP:matrix.org/$gb32QeAGPqopUPI2zlQTC7MHV8PAJYsrMKTmqPIh-LM?via=matrix.org&via=libera.chat&via=matrix.breakpointingbad.com

Worst case scenario people can continue to send links to the Wayback Machine archive https://web.archive.org/web/20221107150752/https://levans.fr/shrink-synapse-database.html but it would be nice to add the guide to admin_faq.md or somewhere else in the official documentation

[DMRobertson]

TL;DR:

1. manually purge forgotten rooms, in lieu of
2. run the state compressor on sufficiently large rooms
3. reindex and vacuum full

Synapse should do (1) itself ( #4720); we'd like it to do (2) itself too. (3) is best left to the discretion of the database administrator: both REINDEX and VACUUM FULL require exclusive locks on tables. (There is REINDEX CONCURRENTLY, but it comes with several caveats.)

[clokep]

(2) run the state compressor on sufficiently large rooms

we'd like it to do (2) itself too.

See #10857