[Core] Template hook + optional YAML front matter

[jeffpatton1971]

Summary
Introduce ITemplateRenderer (default preserves current layout) and RendererOptions.FrontMatter delegate to emit optional YAML for DocsFX/MkDocs.

Tasks

• Add ITemplateRenderer, BasicTemplateRenderer.
• Add RendererOptions.FrontMatter: Func<TypeDoc, IReadOnlyDictionary<string,object>>?.
• Emit YAML when provided; then render via template.

Acceptance

• Default output unchanged when no front matter provided.
• E2E test: sample outputs valid YAML + content when set.