Update the documentation for update sites

[ctrueden]

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.

[imagesc-bot]

This issue has been mentioned on Image.sc Forum. There might be relevant details there:

https://forum.image.sc/t/requests-for-creating-imagej-update-sites/40051/144