[Docs] Partition Doxygen Docs

[matekelemen]

Partition Doxygen documentation into core and each application separately for faster generation.

[matekelemen]

To every application maintainer:

Each application now has its own self-contained doxygen generation and can customize its appearance. You'll find your doxygen configuration file at <kratos_repo_root>/applications/<your_app>/documents/doxygen, which you're welcome to edit to your liking.

Public Applications

To quickly generate the docs and check the results, run the script at <kratos_repo_root>/documents/mkdocs.sh:

mkdocs.sh -a your_app

This generates docs for your_app (provided it is in the kratos applications' directory) as well as core.

Private Applications

If your app is private or otherwise outside the kratos app directory, you can still use the same script, but you'll need to provide the path to the containing directory of your application:

mkdocs.sh -e your_app_name,your_apps_parent_dir

[matekelemen]

this PR is ready @sunethwarna