[Doc] Add CNAME file to preserve custom domain across deployments

Accessing https://ruby.sdk.modelcontextprotocol.io/ is expected to show
the content of https://modelcontextprotocol.github.io/ruby-sdk/,
but it currently does not. This change fixes that issue.

The `generate-gh-pages.sh` script cleans all files from the gh-pages branch root
before copying from `docs/`. This removes the CNAME file that GitHub Pages uses for
the custom domain, causing `ruby.sdk.modelcontextprotocol.io` to break on every release.
https://github.com/modelcontextprotocol/ruby-sdk/blob/v0.10.0/bin/generate-gh-pages.sh#L91-L93

By placing the CNAME file in `docs/`, it will be automatically copied to `gh-pages` during each deployment.
https://github.com/modelcontextprotocol/ruby-sdk/blob/v0.10.0/bin/generate-gh-pages.sh#L95-L97

The following is a PR related to the configuration of ruby.sdk.modelcontextprotocol.io.

- #181
- modelcontextprotocol/dns#14
- modelcontextprotocol/modelcontextprotocol#2486

Author: koic

docs/CNAME

@@ -0,0 +1 @@
+ruby.sdk.modelcontextprotocol.io