docs: link yMMSL configuration reference

[ColumbusLabs]

Closes #190.

Add the live stable yMMSL documentation as a labeled external entry in MUSCLE3’s API documentation navigation, making its configuration examples and Python API reference directly discoverable.

Validation:

• live target returns HTTP 200 and matches the existing yMMSL intersphinx base
• external Title toctree syntax passes an isolated Sphinx 8 build with warnings as errors
• git diff --check

The full repository docs build reaches the existing Doxygen prerequisite and stops because doxygen/json-glib are not installed locally; the changed navigation entry was validated independently.

[carschno]

Hi @ColumbusLabs,
(rephrasing my previous comment)
your contribution to this project is very welcome! We have been wondering about your motivation, and the role of AI in this, for reasons of transparency.

[LourensVeen]

Looks good, and it works, which is always nice 😄. I think the examples and explanation is probably more important than the ymmsl API though, the latter isn't used much by end users, only by tool developers. So I'd prefer to have the link after models_as_software, and maybe just call it "yMMSL documentation"?

[LourensVeen]

Or maybe you've contributed to MUSCLE3 before, under a different account?

This project seems to be completely unrelated to anything else Columbus Labs does, so I'm a bit confused too, but always happy to take contributions 😄. I have added a small request in my review.

[ColumbusLabs]

Thanks, agreed. I moved the link into the main Contents section immediately after models_as_software and shortened the label to “yMMSL documentation” in ea96615.

[ColumbusLabs]

This is my first MUSCLE3 contribution, and I have not contributed under another account. Columbus Labs is working through public open-source issues, and I chose #190 because it was a clearly scoped documentation gap. I used an AI coding assistant to help inspect the repository, make the change, and run the checks; I reviewed the documentation change and validation before submitting.