Information architecture: collapse table of contents?

[stichbury]

This is a child of #1866

We know the table of contents is too long in the documentation, and there will be various activities to reorganise the structure and slim down the sections/pages.

An additional suggestion from @amandakys was to attempt to contract it to sections that can be expanded as needed, rather than show section + child pages.

I know we can do some of this by increasing the complexity of the docs through hierarchy (using embedded .rst files within the main .rst or the MyST equivalent). I'd like to give this a try for a release to see what feedback we get (if any).

[stichbury]

Now merged for us to release and use as a baseline for user research. See #2480 for more details.