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.
Suggest adding to #3:

The URL where the banner should link to goes in the box marked "Destination". Example: https://developer.mozilla.org