chore(python): include docs/UPGRADING.md in docs/index.rst if it exists

[parthea]

PR #1114 added docs/index.rst to python templates. In PR #1114, the migration section was included in the templates. Instead of including the migration section, I'd like to add a kwargs to signal whether a migration guide should be included in docs/index.rst.

I've included the steps I used for testing after cloning synthtool.

git checkout include-upgrading-rst-in-index-rst
docker build -f docker/owlbot/python/Dockerfile -t testindexrst .

After cloning python-access-approval, I completed the following steps

1. Run rm -rf owlbot.py to delete owlbot.py to avoid having to customize owlbot.py to autogenerate docs/index.rst
2. Run docker run --user $(id -u):$(id -g) --rm -v $(pwd):/repo -w /repo testindexrst
3. Confirm that docs/index.rst does not include docs/upgrading.rst
4. Run touch docs/UPGRADING.md
5. Run docker run --user $(id -u):$(id -g) --rm -v $(pwd):/repo -w /repo testindexrst
6. Confirm that docs/index.rst includes docs/upgrading.rst