Document the minimal external project contract

[alonshoa]

Problem

External-project support works when a compatible project and catalog already exist, but the public docs do not show the complete minimal project contract. The clearest example currently lives in tests/fixtures/external_project, which users should not have to discover manually.

Scope

• Add a documented minimal external project directory tree.
• Include a complete starter catalog/scenes.yaml entry.
• Include a complete starter Manim scene that imports manim_kit from the runtime package.
• Include the copyable Compose workflow from docs/compose.external.yml.
• Explain required generated/output directories: builds/, media/, slides/.
• Explain how the external project can use Studio CLI and MCP without copying Studio source code.

Acceptance Criteria

• The docs show every file needed for a minimal external project.
• The documented example aligns with tests/fixtures/external_project or an intentionally promoted equivalent fixture.
• studio doctor --catalog, studio list, studio catalog validate, and studio render demo/smoke --profile draft are copyable for the example.
• The docs state that project generation/installer support is not implemented yet.
• mkdocs build --strict passes.

Dependencies

Uses current Docker runtime and external fixture behavior. Blocks honest external-project onboarding until completed.

Category

documentation, external-projects