emacs: hello: promote '?' as the universal help key

emacs: hello: promote '?' as the universal help key

Move the brief help text at the bottom of the hello screen to the
notmuch-hello-mode help, and promote '?' as the universal help key
across Notmuch. This unclutters the hello screen, and allows for a
more verbose description in the mode help. Hopefully, this change is
useful for both experienced and new users alike.

While at it, improve the links to Notmuch and hello screen
customization.