docs(registry-wg): remove @tadasant from Registry WG Leads (#3175)

* docs(registry-wg): remove @tadasant from Registry WG leadership

@tadasant is stepping down as Registry WG co-lead; @rdimitrov becomes the
sole Lead. @tadasant remains an MCP maintainer and a Registry WG member,
and stays champion of the Server Card / server.json alignment work item.

Leadership-only change: the Membership row moves from Lead to WG Member
and the Emeritus table is untouched.

* Update registry.mdx

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Co-authored-by: Tadas Antanavicius <jon@tadasant.com>