API docs for user preferences

[brontolosone]

Towards #1211

I've hauled it through the sphinx circus to make sure it gets rendered.

[ktuite]

I feel like this shouldn't be a top level section of the Central API, but should be nested under the Accounts and Users section, probably after Users > initiating a password reset, and before the App Users subsection.

You could also mention this in the 2024.3 section of the changelog. There is already one bullet point there.