doc: build mon_command_api.rst using a homebrew extension

doc: build mon_command_api.rst using a homebrew extension

so we can build the command doc using ReadTheDoc infra, without adding
the generated rst file to the source repo.

before this change, all the commands are ordered alphabetically. after
this change, command docs are generated by two directives, and are
ordered separately. we could restructure the directives and merge them.

but let's leave it for a future change if this is important.

for more details on writing sphinx directives, see
https://www.sphinx-doc.org/en/master/extdev/markupapi.html and
https://docutils.sourceforge.io/docs/howto/rst-directives.html

Signed-off-by: Kefu Chai <kchai@redhat.com>