Refactor documentation

[huwb]

This does a major pass on documentation. Moves structure from system-centric (like grouping all loddata types together) to user-centric (grouping things by category, focusing around things users want to do).

This has the high level structure in place and is ready for a high level review i'd say (but detailed notes welcome if you have them).

Docs build: https://crest.readthedocs.io/en/refactor-docs/user/quick-start-guide.html

[daleeidd]

If you rename files you'll likely need to go through the C# code and update links there too.

[huwb]

Thanks yeah there are a couple that differ between file and section header. The ones I changed were more to keep it tidy. But if you want some of them dropped lmk. If not i'll update the C# - good shout!

[huwb]

Pheww think URLs updated although will need to do another round of testing before merge