Embed elisp.py from flycheck, use it in notmuch-emacs doc

[David Bremner <david@tethera.net>]

After finding myself spending a while trying to re-create one of the
features [0] of the el sphinx domain used in flycheck, I decided to try
just converting the docs to use that sphinx extension.

As I remarked in 2018 [1], there doesn't seem to be a distributed
version of this sphinx extension. It also seems like the use-case in
notmuch is a bit different in terms of what we want to put in the
index, but perhaps someone better at sphinx than I could do those
customizations without modifying the extension (as this series does).

Overall I think the quality of the resulting HTML documentation is
quite nice. The change in the info documentation is less clear.  This
may also be tweakable, but in the first version the info docs seem
more verbose than necessary (e.g. writing "Key Binding" everywhere) On
the other hand, if it encourages people (e.g. me) to document things,
then it's probably a win overall.

[0]: documenting both a keybinding and a function name with the same
directive, with nice index entries.

[1]: id:20180521200856.17103-1-david@tethera.net