Document banner uploads

Updates the docs in a few helpful ways:

- Uses the RTD theme for local doc building so we know what it will look like.
- Separates docs into user vs developer guides.
- Adds an intro explaining some basic terminology about the site.
- Rewrites the banner upload docs to match the current site and be more explicit on what to do.
- Adds some help text to the admin.
In the "Image banner variations" section, add the preview....