Content: page for RTD feature comparison to other platforms

[agjohnson]

This came up in a meeting with @benjaoming today. Instead of making this point on a few unrelated pages, we probably benefit from having a consolidated page describing what RTD does compared to other platforms. If we wanted to be heavy about it, we might name other hosting platforms directly, but I also think it's safe to describe missing features of other platforms without directly referencing them.

[pradyunsg]

As an example of another tool in the static site generator space: https://www.gatsbyjs.com/features/

They straight up name the “competitors”/alternatives though.