Prepare docs for modelcontextprotocol.io

[jonathanhefner]

This PR reorganizes and extends the existing documentation in preparation for publishing on https://modelcontextprotocol.io/.

The current plan is for docs to remain in this repo, and be synced to the modelcontextprotocol/modelcontextprotocol whenever necessary (e.g., accompanying a release of the registry). I have included a script in modelcontextprotocol/modelcontextprotocol#1725 (bin/pull-registry-docs.sh) that can be run manually to perform the sync.

If we later decide to permanently move the docs to the modelcontextprotocol/modelcontextprotocol repo, that also works.

This PR puts all https://modelcontextprotocol.io/ docs into docs/modelcontextprotocol-io/. Docs outside that subdirectory, such as docs/administration/admin-operations.md, are intended to be accessible solely via this repo. One major exception is the docs/reference/server-json/ subdirectory, which will depend on the resolution of modelcontextprotocol/modelcontextprotocol#1649. However, I think we could address that in a follow-up PR.

[jonathanhefner]

CI fails with:

Validating docs/modelcontextprotocol-io/remote-servers.mdx: found 6 examples

  Example 4 (line 75):
    Validating against server.schema.json: ✅
    Validating with Go Validator: ❌
      Error: remote URL https://email.example.com/mcp does not match namespace io.github.username/email-integration-mcp: remote URL host email.example.com does not match publisher domain username.github.io

  Example 6 (line 123):
    Validating against server.schema.json: ✅
    Validating with Go Validator: ❌
      Error: remote URL https://email.example.com/mcp does not match namespace io.github.username/email-integration-mcp: remote URL host email.example.com does not match publisher domain username.github.io

My understanding is that there are no restrictions for the remote URL of a io.github.* namespaced server:

registry/docs/guides/publishing/publish-server.md

Line 422 in 18cde88

- For `io.github.username/*` namespaces: No URL restrictions (but you must authenticate via GitHub)

The purpose of the above examples is to demonstrate that.

Is CI incorrect, or do we not currently support that usage?

[jonathanhefner]

Is CI incorrect, or do we not currently support that usage?

Related issues: #494 and #739