docs(community): move Enterprise IG charter into interest-groups nav

PR #2626 landed at docs/community/enterprise-ig/charter.mdx after the
charters were reorganized into docs/community/interest-groups/, so the
page was not listed in docs.json navigation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019cuvoMUPxoYCpTVuroCxMq