Improvements to edit_on_github extension

*Originally requested by @embray in https://github.com/astropy/astropy-helpers/issues/211*

In https://github.com/astropy/astropy/issues/4361#issuecomment-167673551 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.

Start by locating the edit_on_github Sphinx extension and its html_page_content entry point, then inspect handling of a None doctree and the edit_on_github_skip_regex and automodapi_toctreedirnm configuration options. Done means non-document HTML pages no longer crash and API pages are skipped through an updated default or compatible automodapi behavior.

Written by the indexing model from the issue text.