DOC: Clean up of API doc sections
Follow up of the splitting of `api.rst` in #24462.

- Making better use of the titles and subtitles (in date offsets, the class name, and the properties and methods sections where at the same level for example).
- We were inconsistently using `Attributes` and `Properties`, left just properties, as it was the most common one.
- Renamed `indices.rst` to `indexing.rst`, I think it makes more sense (`index.rst` is kind of reserved to the home)
