docs: publish user docs with mkdocs

[CMLKevin]

Summary

• add mkdocs.yml so the docs directory builds as a MkDocs site
• update the publish-docs workflow to build and publish ./site instead of pushing raw ./docs
• add docs/index.md as the user-docs homepage with a pkg.go.dev API reference link

Testing

• python3 -m venv .tmp-mkdocs-venv && . .tmp-mkdocs-venv/bin/activate && python -m pip install mkdocs && mkdocs build
• git diff --check

Fixes #835

[maciej-kisiel]

Could you take a look at the files defined for Python and Java:

https://github.com/modelcontextprotocol/python-sdk/blob/main/mkdocs.yml
https://github.com/modelcontextprotocol/java-sdk/blob/main/mkdocs.yml

We don't have to replicate every customization they have, but they should contain a good baseline for the configuration file – especially look for things that are defined in both.