feature: Extend `show_category_heading` to scopes `mod` or `class`

[Ivernoerve]

Is your feature request related to a problem? Please describe.

Documenting a module with show_category_heading flag renders headers on module and class level.

When rendering a module with classes this leads to deeply nested headers. In many scenarios it could be desired to only show the headers on the class level and not on the module level. This granularity is currently not available.

Describe the solution you'd like

Extend show_category_heading with the following (in addition to existing boolean flags):

• mod
• class

When mod is selected, only objects of kind module shows the category headers, similar for class.

Describe alternatives you've considered

The implementation can be made more flexible by implementing something akin to the summary implementation, with a flag for each desired type to render headings for.

As far as I can see the headings are relevant for a module or class, or both (which is covered by true already) Thus it might be to verbose with such an implementation yet. Open for discussion on this!

Additional context

A PR has been linked with a proposition for how ths can be implemented with respect to the config, as well as an example using the template for the mkdocs materials template.