document the component model

[joshua-temple]

Problem / why

The current docs describe per-component versioning as a reserved, behavior-free shape. Once the
component model ships, the docs must describe the real components: model under
schema_version 1: per-component version, tag, and promotion behavior, and the framing that a
manifest with no components: block is one implicit default component whose output is
byte-identical to the pre-component baseline.

Scope (firm intent)

• Update the reference and explanation docs to cover the components: block, per-component
  version lines and tag prefixes, per-component triggering, per-component promotion, hotfix,
  rollback, and release, and environment subsets. The docs-IA mapping work item decides the
  exact page placement first; the working targets are reference/manifest.md (the components:
  schema), reference/versioning.md (per-component version lines and tag prefixes),
  reference/generated-workflows.md (per-component files), a component-focused how-to guide
  under guides/, and the start/how-it-works.md and internals/architecture.md explanation
  of the component-major model as the native schema.
• Frame single-component as the implicit default: no components: block means one default
  component, and its output is byte-identical.
• 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/ (reference, guides, start, and internals pages per the docs-IA
  mapping). Note the per-component versioning content now lives at
  docs/src/content/docs/reference/versioning.md.

Tests and coverage required

• Docs build and link check pass.

Acceptance criteria

• Docs describe the shipped component model with no remaining reserved-shape language.
• Docs frame single-component (no components: block) as the implicit default with
  byte-identical output.

Dependencies

Blocked by the docs-IA mapping work item and by the independent hotfix, rollback, and release
milestone (so docs reflect the complete lifecycle).

[joshua-temple]

Implemented and merged: the component model is documented in the how-to guide, the manifest and versioning reference, and the how-it-works explanation. PR #531.