feat: update MCP registry docs page with link to main docs

[domdomegg]

Is your feature request related to a problem? Please describe.
Currently at https://registry.modelcontextprotocol.io/docs it has good reference API documentation, but is a bit sparse otherwise. It's not super clear e.g. what the registry is, how to use it.

Describe the solution you'd like
Link to the docs from this page

[tadasant]

Would it make sense to redirect registry.modelcontextprotocol.io to https://github.com/modelcontextprotocol/registry/tree/main/docs for now? Basically until we build a UI for registry.modelcontextprotocol.io directly. Might be easier than adding a good-UX link to the OpenAPI /docs.