Simplify structure

[s-makin]

The auto-generated structure was overly complex, but also not structured enough at the same time. To simplify, I have:

• Added topic folders under each Diataxis level to group content by topic
• Moved landing pages out of "subsection" folder into relevant diataxis folder
• Simplified file names and anchor refs where possible
• Fixed all cross-references to those file names to point to the reference instead
• Added auto-generated subheading anchors, so we can refer to sub-headings by without having to create anchors in the page

To reviewers
This PR should simplify the docs structure as we already discussed during the doc sprint. Since this PR necessarily changes a lot of small things all over the place, I recommend focusing on the spirit of what it's changing - if there are ways we can simplify it further, or you have strong preferences about filename structure, please let me know. Since the filenames affect the resulting URLs, I'd like them to be changed as little as possible once we're live.

Not all links pointing to files have been changed - only the ones affected by my renaming/moving of files (this is intentional).