notmuch-report: Add notmuch-report(1) and notmuch-report.json(5) man pages

To describe the script and config file format, so folks don't have to
dig through NEWS or the script's source to get that information.

The Makefile and conf.py are excerpted from the main doc/ directory
with minor simplifications and adjustments.  The devel/nmbug/ scripts
are largely independent of notmuch, and separating the docs here
allows packagers to easily build the docs and install the scripts in a
separate package, without complicating notmuch's core build/install
process.