Content: add focus for commercial hosting to Sphinx page

[agjohnson]

We highlight embedded documentation as a platform feature above the fold of this page, but embedded documentation is not commonly used, and the most used platform feature we offer is authentication, which isn't mentioned at all.

This is the page that we're hoping attracts users that are not familiar with Read the Docs but who have a Sphinx project. This seems very relevant to this page and to this reader.

We do need a separate page describing authentication, or perhaps documentation is the best place to be focusing those efforts still. So the Sphinx page doesn't need to get that deep into this argument, but I would make sure authentication is prominently highlighted on this page.