This commit creates a Frequently Asked Questions document for our users
to read. Hopefully they actually read it too. Items included are:
Black's non-API, AST safety, style stability, file discovery, Flake8
disagreements and Python 2 support. Hopefully I've got the answers
down in general.
Commit history before merge:
* Create FAQ
* Address feedback
* Move to single markdown file
* Minor wording improvements
* Add changelog entry
* Make the sidebar navigation scrollable
This is necessary since we have so many documentation sections that even
on a desktop screen, the navigation can sometimes be clipped. The only
annoyance is that on Firefox, the scrollbar can't be hidden :(
* allow the docs to build
