[Positioning] Publish public roadmap

[khelanmodi]

Goal

External visibility into what's shipping when.

What to ship

A ROADMAP.md at repo root (or GitHub Projects board, public), with:

• Public preview milestones (what's in / out).
• v1 / GA criteria.
• Known gaps deliberately deferred.
• Link to docs/release-readiness-check.md and docs/gaps-on-rbac.md for the technical detail.

Group by:

• Preview (current): what's shipping.
• v1: blocking GA.
• Future: tracked but not committed.

Acceptance criteria

• Updated at every release.
• Reviewed by PM.
• Linked from README and from aka.ms/documentDBMCPToolkit landing.

Depends on

• All four Positioning Decisions.