Doc additions

[andreww]

A respin of https://github.com/GreenScheduler/cats/pull/139 - I've set up read the docs which supports documentation generation on PRs. As this adds (lots of text) to the documentation we probably want that working for useful review.

The original PR said:

We've been asked to shorten the JOSS manuscript. I think the best bet is to remove the "how this works" material from the manuscript and put it into the documentation. This PR captures that work.

Thus far, this is just text lifted from the manuscript and converted into restructured text (if I have that correct).

[andreww]

I think this one is probably best wrapped up after the JOSS manuscript is out. I think the material makes most sense in a documentation section that starts something like:

A brief outline of the motivation, approach and implementation of CATS can be found in a short paper published in the Journal of Open Source Software. We expand on that documentation here outlining a range of alternative approaches to reducing the carbon cost of computing ...

and then cover at least everything that is in the paper rather than just the stuff we chopped out. While we are at it, we should work in a reference and link to the Turing Way (see #135)

[abhidg]

I'd add some subheadings, also fig1.png is missing

[andreww]

Just adding a note here to check links in the various README files before this lands and make sure they point to readthedocs

[andreww]

Also see #163 - we may as well do everything in one go (when I get time to look at this)