supported-formats: make ratings more compact

This is similar to how it was on the LOCI version of the site, and is
IMO much prettier and easier to read, since the ratings take up much
less space.
Add "make plone" target to sphinx docs makefile

This builds the HTML docs with Plone theme enabled, and all warnings
flagged as errors. (In other words: -W -D html_theme=plonematch)

This target is useful for verifying more precisely how things will look
when embedded within the Plone site. For example, without the plonematch
theme enabled, the sorttable functionality does not work.