Move distributed docs up into docs.dask.org - kill distributed.dask.org

[jsignell]

@jacobtomlinson, @ncclementi and I have been planning for a while to move all the docs from distributed up into dask. This came out of work on #170

The goal of this work is to make distributed docs more discoverable and to reduce duplication.

As far as I can tell these are the parts that still need to move:

• [ ] Top level API reference
• [x] Dashboard narrative docs
• [x] Deploying clusters narrative docs
• [ ] Move narrative client/scheduler docs into https://docs.dask.org/en/stable/scheduling.html section
• [ ] Move any other useful docs into https://docs.dask.org/en/stable/internals.html or https://docs.dask.org/en/stable/how-to/index.html