Extend documentation

[a-zw]

Open points:

• persistency has no needs in docs yet, thus breaks :docs build
• communication has no documentation yet
• orchestrator doc build breaks
• nesting collections is a problem (docs-as-code tries to use _collections recursively)

[a-zw]

Interesting error in CI because locally I don't see it:

/home/runner/work/reference_integration/reference_integration/docs/_collections/score_platform/docs/requirements/index.rst:18: WARNING: duplicate label requirements, other instance in /home/runner/work/reference_integration/reference_integration/docs/_collections/score_docs_as_code/docs/internals/requirements/index.rst

It is right though: Both repos have a requirements section and define a requirements label there.