Add release compatibility matrix and rewrite support claims

[vgvoleg]

21. Add release compatibility matrix and rewrite support claims

Suggested milestone: M6 Release Candidate

Suggested labels: priority:p0, area:docs, area:release,
type:docs, status:blocked

Context

README and docs currently make broad support claims. Before non-beta, docs
should reflect tested behavior and known limitations.

Work

• Add compatibility matrices for versions, fields, relations, constraints,
  indexes, transactions, migrations, ORM features, introspection, Admin/Auth,
  and UPSERT.
• Rewrite README support claims to match tested behavior.
• Document local YDB setup on ydbplatform/local-ydb:trunk.
• Document test commands, troubleshooting, and cleanup.
• Add release notes template distinguishing supported behavior from known
  limitations.

Acceptance Criteria

• Docs match code and tests.
• A new user can run local YDB, migrate a sample app, and understand
  limitations before production use.
• Public release notes can link to the compatibility matrix.

Dependencies

Blocked by:

• Define public support contract and compatibility matrices
• Audit DatabaseFeatures and align flags with tests
• Add Django contrib smoke test project
• Add migration executor tests
• Improve introspection nullability, sequences, and indexes