Improvements to edit_on_github extension

[embray]

In astropy/astropy#4361 (comment) I suggested a few fixes to the edit_on_github Sphinx extension that might be worth making:

• If the doctree argument passed to html_page_content is None, this implies an HTML page that was not generated from an actual rst document. Usually this implies that there is no page to edit on GitHub, though I suppose a static HTML page that lives in the repository is also possible. In either case, the case of doctree is None should be handled in some appropriate way. Currently if doctree is None for a page that is not skipped, the extension crashes.
• The default for edit_on_github_skip_regex should be updated, probably to include api.* pages for compatibility with automodapi. Better yet, maybe the extension can check if automodapi is being used, and automatically skip anything that starts with the value for the automodapi_toctreedirnm config option used by automodapi.

[astrofrog]

I've moved this issue to astropy/sphinx-astropy#6