doc: fix for out-of-tree builds of notmuch-emacs docs

The sphinx-doc include directive does not have the ability to include
files from the build tree, so we replace the include with reading the
files in conf.py. The non-trivial downside of this is that the emacs
docstrings are now defined for every rst source file. They are
namespaced with docstring::, so hopefully there will not be any
surprises. One thing that is noticable is a small (absolute) time
penalty in running sphinx-doc.