Better documentation link on the list_plugins page

[EliAndrewC]

When running make html in the sphinx directory, Sideboard builds its docs at a location that gets mounted by CherryPy. The list_plugins page has a link to this page.

Unfortunately, most developers will see this link and click it without building the docs, which means that the link is broken in a confusing way.

We should instead upload our Sphinx docs to somewhere like https://readthedocs.org/ and then change the link to point there.