Update Documentation for 5.1.0

[aaronayres35]

targeting [gh-pages]

These are the steps I followed for building the documentation:

git clean -ffxd
git checkout 5.1.0
python etstool.py install
git clean -ffxd
python etstool.py docs
git checkout gh-pages
git pull
git checkout -b gh-pages-5.1
mkdir 5.1
rsync -r --exclude="\.buildinfo" docs/build/* 5.1/
rm -r docs
rm -r `ls -d [![:digit:]]*`
cp -r 5.1/* .
git add .
git commit -m "create new 5.1 folder to hold current docs.  Replace current documentation (not in a folder, which used to match contents of 5.0) to now match contents of 5.1"

To test:
python -m http.server

Checklist
- [ ] Add a news fragment if this PR is news-worthy for end users. (see docs/releases/README.rst)

[rahulporuri]

Tested this locally. LGTM.