guix describe: Display channel introductions and add 'channels-sans-intro'.

* guix/scripts/describe.scm (%available-formats): Add "channels-sans-intro".
(channel->sexp): Add #:include-introduction?.  Emit CHANNEL's intro if
INCLUDE-INTRODUCTION? is true and CHANNEL has an introduction.
(channel->json): Include CHANNEL's introduction, if any.
(channel->recutils): Likewise.
(display-profile-info): Add 'channels-sans-intro' case.
* doc/guix.texi (Invoking guix describe): Add introduction in example.
Add 'channels-sans-intro' case.