docs: autogenerate animations, compare with CI

[kmantel]

Generating animations used in the docs (currently only
Composition_XOR_animation.gif referenced in composition.py) allows
changes to be monitored with the docs-compare CI. Originally mentioned
in #1861, to allow
updating pillow requirement

[github-actions]

This PR causes the following changes to the html docs (ubuntu-latest-3.7-x64):

diff -r docs-base/Composition.html docs-head/Composition.html
870c870
< <a class="reference internal image-reference" href="_images/Composition_XOR_animation.gif"><img alt="Animation of Composition with learning" src="_images/Composition_XOR_animation.gif" style="width: 397.0px; height: 406.0px;" /></a>
---
> <a class="reference internal image-reference" href="_images/Composition_XOR_animation.gif"><img alt="Animation of Composition with learning" src="_images/Composition_XOR_animation.gif" style="width: 328.5px; height: 401.5px;" /></a>
Binary files docs-base/_images/Composition_XOR_animation.gif and docs-head/_images/Composition_XOR_animation.gif differ
Only in docs-base/_static: Composition_XOR_animation.gif

...

See CI logs for the full diff.