help: register the 'gpg' command category and give it a description
help.py expects extensions to register their command category in the
CATEGORY_ORDER and CATEGORY_NAMES variables. Once gendoc.py orders
commands by category, in the next patch, it'll assume this registration
(and raise an exception on encountering any unregistered categories).
Luckily, gpg is the only bundled extension with an unregistered custom
category, so let's fix it.
Differential Revision: https://phab.mercurial-scm.org/D6324