notmuch.el: Implement our own notmuch-help instead of describe-mode.

notmuch.el: Implement our own notmuch-help instead of describe-mode.

This gives somewhat friendlier output for the '?' binding than we had
previously with `describe-mode'. First, we no longer have the various
minor modes cluttering up the output. Second the display of the
binding table uses the first line of documentation for the bound
function rather than the function name.