Initial stab at mode documentation

[ccasin]

This adds some high-level documentation for our modes. The idea is to have a page that gives a brief overview of the whole suite of modes, without getting too far into the details of any one of them (for which it refers to the documentation about that particular feature).

I haven't attempted to write docs about the following axes: statefulness, visibility, yielding. Either someone else can write those and push them here, or they can be added in a future PR.