Update docs to spread knowledge

[DominicOram]

As discussed with @graeme-winter, @noemifrisina, @dperl-dls and myself, we need to spread the knowledge of how nexgen works and what it does. There are already some docs, this is a list of improvements for them.

Nexgen is currently being used for some specific applications at beamlines I19-2 and I24 at DLS.

This doesn't mention Hyperion, ED or how their use differs between each beamline

I think it would be good to have some specific mentions of if each beamline uses the CLI etc.

The autogenerated docs are good for individual parts of the code but don't give a high level view. It would be good to write some kind of code map https://github.com/DiamondLightSource/hyperion/wiki/Code-Map for how to navigate the codebase.