docs(community): Add Enterprise Interest Group charter (#2626)

* docs(community): Add Enterprise Interest Group charter

Charter for the MCP Enterprise Interest Group, co-facilitated by Raghu Chandra and Yannj_Fr. Reviewed by 13 participants from Saxo Bank, Nordstrom, Boomi, Okta, Blue Shield of California, Silex Data Solutions, TraceForce, GNS-Foundation, EmpowerID, Solo.io, and Archestra.

* Update docs/community/enterprise-ig/charter.mdx

Co-authored-by: Joey Orlando <joseph.t.orlando@gmail.com>

* Tighten scope to requirements-gap focus per review

* Fix markdown formatting (prettier)

* Incorporate review feedback: stateless transport, scope, related groups

* Add opportunity framing to In-Scope per review

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Co-authored-by: Joey Orlando <joseph.t.orlando@gmail.com>
Co-authored-by: Ola Hungerford <olahungerford@gmail.com>