Reconfigure RTD to show individual stretch/jessie versions

[WasabiFan]

At the moment I can't figure out how to create a new "version" for stretch, and the Jessie version is erroring because of that missing submodule revision.

[WasabiFan]

I've mostly gotten this figured out now, in that there are ev3dev-jessie, ev3dev-stretch-1.x and ev3dev-stretch versions. Currently, it doesn't seem that tags automatically show up in the version selector, so as we release them from here forward I'll activate them on RTD.

[WasabiFan]

The v2 beta has been published and I activated the tag for that release for historical reference.