Add blog post announcing Ruby SDK 1.0

Announce the first stable release of the MCP Ruby SDK: a stable public API
under the Semantic Versioning policy, a 100% server and client conformance
pass rate, and the Tier 2 assessment (#3127).

If accepted, this post is intended to be published after #3128, which moves
the Ruby SDK to Tier 2 in the official SDK listing, is merged.