Extensible metamodel

[a-zw]

When reusing docs-as-code for internal/proprietary purposes, I need to extend the metamodel and its checks.

While it is possible to some degree with some Python hackery, there should be a clean and documented way to do this.

The current metamodel extension also applies checks to types not defined for the S-CORE metamodel. For example, the id schema is enforced for any type.

[MaximilianSoerenPollak]

Here i think we should look into if we want to answer the fundamental question:

do we want to split docs-as-code internally.
Meaning that we make it a 'general' solution that can accept metamodels & tests etc.

Or do we just want to make it so an additional metamodel can be supplied without splitting docs-'s-code internally too

[a-zw]

I think this metamodel.yaml is a nice way to specify it. At least, it is better than this schemas.json approach from Sphinx-Needs. Though I can image that @ubmarco might disagree.

Anyways, we should probably not restrict users, so we need an agnostic approach to merge metamodels? In the end, it is just appending objects to app.config.needs_extra_links in a Sphinx extension.