Fix formatting and cleanup (#593)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/593

- Added a check to the custom role to ensure sphx-gallery outputs are not affected.
- Removed template tutorial custom card from index.rst
- Don't generate tutorials/index.html - because sphinx autogenerates a page with all tutorials that is hard to format.
- Added sys.path so we can use autodoc in .md files.

Reviewed By: mergennachin

Differential Revision: D49878715

fbshipit-source-id: b9f649b17d81dca4879ca82887a6574bb8c33030