Document the hub-aware announcement and soften the spec-major coupling

[lorisleiva]

This PR aligns RELEASING.md with codama-idl/release-tools#7 and a clarified invariant:

• The announcement thread always lives in the ecosystem hub (this repository's Discussions), whichever repository drives the transition; its copy names the driving repo.
• Independent majors with ecosystem impact (e.g. breaking dynamic-* behaviour in the codama monorepo, without any spec change) run the full candidacy with an announcement, not the fast-track.
• The monorepo same-major invariant is one shared major per monorepo; coinciding with the spec major is today's convenience, not a rule.