BUG: Fix list of possible docstring section header patterns for global_enable_try_examples

[steppi]

Closes #256

This PR updates the list of section header patterns for determining the next section after the examples section to a list which should work for sphinx.ext.napoleon. It also updates the list of possible numpydoc section patterns for correctness. The latter doesn't really matter that much though. Recent versions of numpydoc are opinionated and will re-arrange headers to fit a preferred order, so most of these patterns will never be found after an examples section after preprocessing by numpydoc. It wasn't much extra work to include the correct pattern for all headings though.

The core idea here is that sphinx.ext.napoleon and numpydoc each map human writable headers like

Examples
--------------

to directives and other forms of rst markup, and there are some differences in what they can output for different section headers, but this wasn't accounted for because I had only tested global_enable_try_examples with numpydoc. sphinx.ext.napoleon is also widely used though (e.g. Astropy, IPython, XArray) and should be supported.

[agriyakhetarpal]

Thanks, @steppi! I have a suggestion and a question based on our original plan for what contents are to be included. Also, I think it would be nice to link a "Kitchen Sink" sort of page in our docs with the TryExamples directive, containing all the relevant options from your toy/test website that you mentioned earlier in our meeting, even if it's not added to an index anywhere and stays as an orphan page, so that we can see on further PRs that change these files that might that things remain working – considering that we virtually have no tests.

Also, the tests are failing, I think we ought to add black as a pre-commit hook instead soon

[steppi]

Thanks @agriyakhetarpal!

[agriyakhetarpal]

Thanks, @steppi!

[agriyakhetarpal]

I would like to merge this for now so that it can unblock further work on the related issues and reduce conflicts with my work to make notebook modifications more generic. We can work on other improvements before the release in case we might have missed anything. Thanks for improving this area of the code, @steppi!