Docs: Argument Clinic: Add Background and Tutorial top-level sections (#106904)

Docs: Argument Clinic: Add Background and Tutorial top-level sections (#106904)

Add Background as a toplevel section with the following subsections:

- Background
  - The goals of Argument Clinic
  - Basic concepts and usage

Rename "Converting your first function" to Tutorial.

Add anchors for Background, Tutorial, and How-to Guides:

- :ref:`clinic-background`
- :ref:`clinic-tutorial`
- :ref:`clinic-howtos`

Link to these from within the Abstract.

Break the compatibility paragraph out of Abstract and make it a note.

Co-authored-by: Ezio Melotti <ezio.melotti@gmail.com>