Expand README with repo layout and setup steps

[sid-rl]

Proof-of-concept PR for webhook testing.

The README was a bare # sandbox stub. This fills it in with a table of what lives where, uv setup steps, and the Sphinx docs build command. Docs-only — no code changes.

🤖 Generated with Claude Code

[rlagentflowbeta]

✅ Reflex agent status: Completed

Agent	Status
agt_5CLs35Ycep1BjU6gUbpske	✅ Completed

This PR was created by Reflex.

_{This comment updates in place as agents work.}