docs(uv-workspace-starter): add maintainer-facing bank README.md

[ulises-jeremias]

Context

templates/uv-workspace-starter has a generated-project README.md.template but no maintainer-facing bank README.md. Other templates document Apply / Verify for contributors.

Steps

1. Read an existing extension or template README for pattern (e.g. extensions/fastapi-docker/README.md or another template README).
2. Create templates/uv-workspace-starter/README.md with Apply and Verify sections.
3. Pull compatible extensions from templates.json.
4. Keep generated-project docs in README.md.template — do not replace that file.

Acceptance criteria

• New maintainer README exists at templates/uv-workspace-starter/README.md
• Includes Apply command using uvx create-awesome-python-app ... --template uv-workspace-starter
• Includes Verify steps (uv sync, uv run pytest or Makefile targets)
• Lists compatible extensions from the registry
• Does not overwrite README.md.template

Files

• templates/uv-workspace-starter/README.md (create)
• templates.json (reference)
• templates/uv-workspace-starter/Makefile (reference)

How to verify

Follow Apply/Verify from a clean temp directory using the documented commands.