- Keeps the migrations to delete the public tables
- Updates and adds a global footer (admin configurable)
- Update the layout of homepage

Fixes #3110 #3559


## Test Steps

- [ ] Remove the public site, go to "Wagtail Admin" > "Pages", set the
"Apply Home Page" as the default page of the site. Remove the public
site page
 - [ ] Do regression of the whole site

---------

Co-authored-by: Wes Appler <wes@opentech.fund>

- Keep the app installed so the delete table migrations could run
- Update sandbox-db with removed tables

Depends on #3715

The workflows page in the docs had duplicate info about each of the work
with just an addition screenshot of workflow stages section.

Update and merges the screenshot with the existing sections about
different workflows

The images are linked relative to page now. This prevents the mkdocs
warnings and also let these images render itself nicely inside the
editor or github interface.

This way of referencing also helps for future if the docs get deployed
to a path instead of a top-level domain

Fixes #3567

- Add page last edited info on each page
- Update header - site title, repo name & icon
- Disable custom fonts, use system-fonts
- Add copyright info in footer
- Update navigation structure and behaviour
- Add source code view and edit buttons
- Refractor documentation