Update / improve documentation

[glass-ships]

It seems like the existing documentation may be somewhat out of date, or may have some areas which can be improved, particularly where installation is concerned.

We may also want to double check for and look over (or create) a section/page for developing/contributing

[bmaranville]

I like "somewhat out of date". It's very kind.

[bmaranville]

And there is indeed a contributing page, but it's maybe a little hard to find: https://github.com/reflectometry/refl1d/blob/master/doc/getting_started/contributing.rst

I make this statement with evidence, since the developer who just reorganized the entire codebase still didn't find it.