docs(onramp): add a project manifest starter example to docs/site/reference

[flyingrobots]

Summary

Add one short project-manifest example to docs/site/reference.md that links to docs/reference/project-manifest.md and shows the current JSON/YAML manifest path.

Acceptance Criteria

• docs/site/reference.md links to docs/reference/project-manifest.md.
• The example uses wesley.config.json, not retired wesley.config.mjs.
• The text stays domain-free and does not introduce downstream target semantics.
• cargo xtask docs-check passes.

Notes

This is a scoped starter docs task for the contributor onramp.