Remove live-workshop resources footnote after devcon (rotate sandbox creds + Zulip invite)

The root README has a **Live workshop resources** footnote (added in #12) with two temporary, publicly-visible links:

- Sandbox credentials: `https://sandbox.assets.juspay.in/test/devcon/creds.env`
- Zulip collaboration channel invite: `https://potentially-invisible-gigs.zulipchat.com/join/477ekeq2niffdt4rfqco6z2a/`

These are meant for the live devcon session only. Since this is a public repo, they're exposed to anyone who finds it.

**After the session:**
- [ ] Remove the "Live workshop resources" footnote from `README.md`.
- [ ] Rotate / revoke the sandbox keys behind `creds.env` (and take down or empty the asset).
- [ ] Expire or revoke the Zulip join link.

Reference: footnote added in #12.

Start in README.md and inspect the “Live workshop resources” footnote added in #12, then check the sandbox credentials URL and Zulip invite listed in the issue. Done means the footnote is removed, the sandbox keys are rotated or revoked and the asset is taken down or emptied, and the Zulip join link is expired or revoked.

Written by the indexing model from the issue text.