Improve documentation on becoming server admin (#13230)

* Improved section regarding server admin

Added steps describing how to elevate an existing user to administrator by manipulating a `postgres` database.

Signed-off-by: jejo86 <28619134+jejo86@users.noreply.github.com>

* Improved section regarding server admin

* Reference database settings

Add instructions to check database settings to find out the database name, instead of listing all available PostgreSQL databases.

* Add suggestions from PR conversation

Replace config filename `homeserver.yaml`. with "config file".
Remove instructions to switch to `postgres` user.
Add instructions how to connect to SQLite database.

* Update changelog.d/13230.doc

Co-authored-by: reivilibre <olivier@librepush.net>