Feat/group by category increase granularity

[Ivernoerve]

For reviewers

• I did not use AI
• I used AI and thoroughly reviewed every code/docs change

Description of the change

Updating config for group_by_category to increase granularity with mod and ´class´.

To demonstrate how the change can be implemented, the material children.html.jinja template has been altered with an implementation that accounts for the granularity.

TODO

• Correct header in docs (I have to correct the type as it is extended to bool | int

Relevant resources

• feature: Extend show_category_heading to scopes mod or class #315