Allow adding extra documentation in Markdown format

[ssbarnea]

While Sphinx itself can use ME instead of RST via MyST sphinx plugin, anstibul-docs is unable to expose markdown files as links (even if sphinx does build the files correctly)

I am referring to https://ansible.readthedocs.io/projects/antsibull-docs/collection-docs/#adding-extra-documentation which documents on how to make extra documentation visible.

This problem is quite important because current collection packaging allows ONLY markdown documentation to be rendered in galaxy/hub, this being the only official way to ship documentation as part of the collection itself.

I suspect that this can easily be addressed by relaxing the check done by antsibull-docs.

[felixfontein]

Currently antsibull-docs only supports RST since we need to make sure that only labels that these documents insert into the global label namespace adhere to the requirements, namely that they are prefixed with ansible_collections.<namespace>.<name>.docsite..

So we need to find out a) what kind of labels MyST documents can insert in Sphinx, and b) how we can enumerate all these labels in antsibull-docs so we can only copy files which use correct labels.

[felixfontein]

https://myst-parser.readthedocs.io/en/latest/syntax/cross-referencing.html seems to describe references in MyST.