rewrite of the `Coordinates` section in the data structures guide

[keewis]

During the index refactor the Coordinates class has become much more important than it was before, changing from a simple proxy class to a full-featured container.

This PR changes the section to focus on the Coordinates class, which makes a bit more sense to me because the entire page is supposed to talk about data structures. However, describing the concept of coordinates in a section that looks a bit more advanced (I'd expect an average user to rarely interact with the Coordinates object directly) might make it easier to miss?

Either way, I'll keep working on this for a while.

• [x] towards #6293
• [ ] User visible changes (including notable bug fixes) are documented in whats-new.rst

[TomNicholas]

However, describing the concept of coordinates in a section that looks a bit more advanced (I'd expect an average user to rarely interact with the Coordinates object directly) might make it easier to miss?

Why not both? Put a brief description here and a link to a more detailed description somewhere else?