README: Extract contribution instructions. Add ToC

[Fixes #56443882]
Try moving build status images onto first headline
Alphabetize the list of supported CIs
Fix broken ToC link
Rename CONTRIBUTE.md -> CONTRIBUTING.md

Github uses the new name to suggest how one might contribute to the
repository. Let's take advantage of it.
Extract Upgrading and 'Adding a project'
Extract display section
Add example screenshot to display.md
Rename docs/display{,ing_your_projects_status}.md
Rename upgrading.md -> upgrading_to_devise.md [ci skip]