Use emacs docstrings in sphinx-docs

[David Bremner]

This series obsoletes the WIP series at

     id:20180527185530.6795-1-david at tethera.net

I think it's ready for discussion and potential merging.

This is a simple tool that loads an elisp file, and extracts all the
variables/functions it defines and the corresponding
docstrings.

[PATCH 01/11] emacs: initial version of rstdoc.el

These are technically bugfixes for the elisp, and could be applied independent
of the rest of the series.

[PATCH 02/11] emacs: require notmuch-lib from notmuch-wash.el
[PATCH 03/11] emacs: escape quote in docstring

There are 3 patches worth of build system glue. In particular, we now
only build the emacs-manual if we have emacs.

[PATCH 04/11] emacs: build docstring (rsti) files
[PATCH 05/11] doc: only build notmuch-emacs docs if Emacs is present
[PATCH 06/11] doc/emacs: require extracted docstrings for sphinx or

The remaining patches in the series use the extracted docstrings to
start filling in the notmuch-emacs manual.