Documentation restructure

[jfrost-mo]

The documentation has been reorganised to better allow distinct kinds of documentation, with distinct audiences, to be written.

This involves splitting the documentation into several distinct sections based on how it should be written. This is Tutorials, How-to guides, References, and Explanations.

• Tutorials are for people learning their way around.
• How-to guides answer the "how?".
• References answer the "what?".
• Explanations answer the "why?".

This is a commonly used, and well regarded documentations style used by projects such as NumPy.

More information can be found here: https://documentation.divio.com/

Fixes #146

[jfrost-mo]

This PR just restructures the documentation. It will be an ongoing effort to expand and update the documentation.

[github-actions]

[jfrost-mo]

To aid review, here is a rendered version of the documentation.

[jfrost-mo]

worth also commenting on improved quality assurance as well as preventing effort on duplicated code. I.e. adhoc scripts in different peoples dirs may have bugs, whereas CSET at least is scientific/technically reviewed code so more likely to catch bugs in diagnostics.

Yeah, that would be a good addition. I’ll add it.