Added plugins docs page

There is now a page docs/source/plugins.rst which will house all of the docs
for orvsd_central's plugins. Currently the file only has information on the
branching scheme of the plugins.
My understanding after talking with @Kennric is that master shouldn't/doesn't need to exist, it's just there from when we had all the versions of the plugins in the same branch. The switch to <moodle-version#>_develop, etc. was to make branching easier/management easier. I'll make some changes to explicitly state this in my next commit.