doc: extensions: samples: Introduce code sample categories

This commit adds support for categorizing code samples in the
documentation.

It introduces two new directives:

- `zephyr:code-sample-category::` to create a category and associated
  brief description, that implicitly acts as a toctree too.

- `zephyr:code-sample-listing::` to allow dumping a list of samples
  corresponding to a category anywhere in the documentation.

Fixes #62453.

Signed-off-by: Benjamin Cabé <[email protected]>