New consolidated docs on RTD

[oberstet]

For various reasons, including the integration of Crossbar.io FX into the OSS repo here, the docs are in a messed up state:

• https://github.com/crossbario/crossbar/tree/master/docs
• https://github.com/crossbario/crossbar/tree/master/docs-old
• https://github.com/crossbario/crossbar/tree/master/docs-cfx

The docs are (for historical reasons) also not on RTD, as are the docs for virtually all other related projects like Autobahn.

Hence, the issue here is to

1. [ ] consolidate above doc sets into one Sphinx based documentation set
2. [ ] add CI to build docs and publish to RTD