[REQ] Update TopSim documentation.

[myxie]

What is the requirement? Please describe.

As said in the title, it makes sense to have all of this in one place.

Describe the solution you'd like

When our documentation is hosted on readthedocs.org, we should be able to 'setup' with SKA workflows, and the simulate with TOpSim.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features.

Additional context
Add any other context about the requirement.

[myxie]

Linking to the discussion in top-sim/skaworkflows#48, I'm going to re-direct the focus of this to actually building individual documentation sources, and just referencing the documentation between the different repos, rather than trying to build a complete set of docs in the one repo.