Fix sphinx build errors
Improve git_pillar documentation

This adds a brief explanation of git_pillar to the gitfs walkthrough.
Even though they don't work the same, they do similar things for
file_roots and pillar_roots, and it bears mentioning.

Also, this adds a better explanation of how git_pillar works to the
top-level docstring for pillar/git_pillar.py.
Merge pull request #8147 from terminalmage/docs

Improve git_pillar docs