Rearrange the docs by area, not CLI vs Python

[simonw]

The docs for eg inserting data should live on the same page, rather than being split across the API and CLI pages.

[simonw]

I've changed my mind on this one. I don't think the docs will read particularly well if they try to cover both of these things at once. I could set up links between the two, but actually trying to combine them feels like it will result in documentation that is less useful.