Update the documentation for update sites

We need to update the docs surrounding update sites to recommend people (at least for now) start a forum topic asking for update sites to be created, plus granting write permission to the needed user(s). MediaWiki-specific instructions surrounding update sites need to be removed from the documentation.

Start by locating the documentation pages covering update sites and read their current instructions. Update them to direct users to request update sites and the required write permissions through a forum topic, then remove the MediaWiki-specific instructions and check the resulting guidance for consistency.

Written by the indexing model from the issue text.