Update website content to more accurately reflect what we support today

Created by: nicksnyder

Relates to https://github.com/sourcegraph/about/issues/5230

Our current homepage lists many things that we could add support for (and can technically be made to work), but not all of these things are equally supported in a first class way. This marketing page served its purpose when we we wanted to attract a broad funnel of customers and then build support on-demand, but this is not going to be our approach moving forward, so we need to clean this up.

On the surface this is "remove all the things we don't support" (like SVN), but after doing that we might actually need to redesign the layout to make more sense.