docs: document schema-2 multi-component manifests

[joshua-temple]

Problem / why

The current docs (e.g. docs/src/content/docs/versioning.md) describe per-component
versioning as a reserved, behavior-free shape. Once schema-2 ships, the docs must describe the
real schema_version 2 components: model: per-component version / tag / promotion behavior,
the additive opt-in-by-format adoption (write a schema-2 manifest; schema-1 keeps working),
the optional cascade upgrade-manifest helper, and the single-service-is-one-component
framing.

Scope (firm intent)

• Update versioning and manifest docs to cover the schema-2 components: block, per-component
  version lines and tag prefixes, per-component triggering, per-component promotion / hotfix /
  rollback / release, environment subsets, and the opt-in-by-format adoption path.
• Document that schema 1 and schema 2 coexist, schema 1 runs unchanged forever, and there is
  no forced migration; mention cascade upgrade-manifest as the optional convert helper.
• Remove the reserved-shape language now that the shape carries behavior.
• State plainly that cross-component dependencies are out of scope and the external /
  notify mechanism is the coordination path.

Files / areas touched

• docs/src/content/docs/ (versioning and manifest pages).

Tests and coverage required

• Docs build / link check passes.

Acceptance criteria

• Docs describe the shipped schema-2 model with no remaining reserved-shape language.
• Docs make the additive, opt-in-by-format adoption explicit (no forced migration).

Dependencies

Blocked by the monorepo: independent hotfix and release milestone (so docs reflect the
complete lifecycle).