Docs: Argument Clinic: Group guides about default values (#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