Replace the quick-start's inline onboarding procedure with a copyable
prompt that points to the canonical Trigger.dev setup skill. Add the
same prompt to the docs introduction, with an explanation of the setup
flow and a link to inspect the full instructions.

Use Mintlify's native code-block copy control and preserve the
quick-start's `#copy-the-setup-prompt` anchor.

Mono-RevId: 251048e2a4f643e9bc255ab33fb7fa2daebf3d5a