This introduces a Settings guide, which covers all the settings.py

settings managed by Djblets. It's broken down into category, using the

setting:: directive provided by beanbag-docutils. As a note, these

docs are initially written by Claude, edited by me.
All references to these in existing docstrings and guides have been

updated to use the :setting: role.

Built the docs. Checked that the settings page's organization and

descriptions were correct. Checked the links from other doc pages

to make sure the links were correct.
Verified there were no build errors.