install info docs v1

[David Bremner]

I was recently reminded we still don't install the info docs for
notmuch-emacs, so I set out to remedy that. The other issue is that
the notmuch-emacs docs are not very good. I found somethings that I
thought would be simple to be rather frustrating in trying to update
notmuch-emacs.rst. That could well just be a lack of fluency in rst on
my part. I did wonder if maybe we should consider using some sphinx
extension for elisp. I didn't find a well maintained and widely
distributed one.  sphinxcontrib-emacs apparently doesn't support
python3 which is a blocker for me.  flycheck embeds an extension [1] that
looks fairly nice. It's GPL3+ so we would in principle fork/embed it.

[1]: https://github.com/flycheck/flycheck/blob/master/doc/elisp.py