Sphinx errors have been fixed in the plugin projects for octavia, rsd,
trove, watcher, and zun, so we can now use autoprogram-cliff to generate
the docs for those.

Change-Id: Ia7790c5e86957afd0aec8f9a04ffc7aa968b4eeb
Story: #1735016
Task: #37241

We would like to use autoprogram-cliff to generate full docs inline for
each plugin. But for the following projects, that breaks the build:

- octavia
- rsd
- trove
- watcher
- zun

For those projects, we're using list-plugins instead, because that
builds, and it's better than nothing; but it only provides summaries of
the commands.

So with this commit, we add a link to the plugin documentation from the
actual plugin project where such documentation exists, which currently
is just:

- octavia
- watcher
- zun

(For rsd, I couldn't find openstack-published docs at all; for trove,
published docs exist, but the osc plugin isn't documented.)

Change-Id: I7c826ecef4319bead239e11b5f975302b2f24d1b
Story: #1735016
Task: #37244

This was being omitted because whenever the plugin page was produced,
python-watcherclient wasn't in global-requirements. It is now, so
include a page for it in the plugin docs.

NOTE: We would like to use autoprogram-cliff to make the documentation
complete, but that breaks the build. For now, this is better than
nothing.

Change-Id: I49822242b9a0c031a053d6c2fd9f644a585f4ba5