[3.12] Docs: Argument Clinic: Group guides about default values (GH-106872) (#106874)

Docs: Argument Clinic: Group guides about default values (GH-106872)

Previous ToC layout (excerpt):

    - How to use symbolic default values
    ...
    - How to assign default values to parameter
      - How to use the ``NULL`` default value
      - How to use expressions as default values

New layout:

    - How to assign default values to parameter
      - The ``NULL`` default value
      - Symbolic default values
      - Expressions as default values

(cherry picked from commit 505eede38d141d43e40e246319b157e3c77211d3)

Co-authored-by: Erlend E. Aasland <erlend@python.org>