As seqdiag, blockdiag and nwdiag blocks in docs are no
longer supported with the latest pillow code, did the
following to fix the docs job:
- Made screenshots of seqdiag/blockdiag/nwdiag images and
removed code that built them, started using the images
- Created *diag files of above for posterity
- Removed unused footnotes in some other files
- Removed unnecessary files
- Removed unused requirements
- Bumped sphinx>=2.2.0 to match neutron repo
Closes-bug: #2089157
Change-Id: Ie9a1a18af4a21057a6cf8380c664fc4d353d2d73
8817b14342