Add Project Wiki (markdown) documentation for Drupal specific drowl_base/child classes

@thomas.frobieter good idea! We simply need to create the markdown filese here in the module, like in the example:
https://git.drupalcode.org/project/project_wiki/-/tree/1.x/modules/proje...

The module will then automatically register them based on a simple plugin file, leading to the directory:
https://git.drupalcode.org/project/project_wiki/-/blob/1.x/modules/proje...
(If we'd like to, we could even replace it by a .yml definition sooner or later)

@thomas.frobieter: For wiki contents you don't need to create a submodule, it's just opt-in.

So only if project_wiki_markdown_content is present, the Plugin will be called. So the code can be put right into the parent module.
I'll move it and implement the required changes.

Automatically closed - issue fixed for 2 weeks with no activity.