chore: bump external/onflow-docs submodule pin (2026-02-11 → 2026-04-27)

Was: 524bdbfe6 (2026-02-11, 'add Devnet54 #1651') — 75 days stale at
the time of bump.
Now: 2ffd11602 (2026-04-27, 'Merge PR #1653 seo-geo-robots-content-
signals-2026-04-23') — current onflow/docs main.

The pin drives federated search content (per the AI-surfaces commit:
src/lib/external-search-index.ts walks external/onflow-docs/docs at
server cold-start to index developers.flow.com pages alongside local
content). With the bump, search results align with what developers
see at the published site rather than a 2.5-month-old snapshot.

R1 (security review) verified: pin is on a real commit reachable from
onflow/docs origin/main, not branch-tracking. Refresh cadence should
become a quarterly manual bump or a monthly auto-PR (tracked at #300).

Author: Aliserag