docs: retarget docs.libp2p.io links to libp2p.io

[imitater-dou]

Summary

https://docs.libp2p.io (and concept paths under that host) currently return HTTP 404. The live docs hub is https://libp2p.io/docs/, with concept pages under https://libp2p.io/concepts/... (same path suffix).

Changes

• README.md: retarget Our documentation link to https://libp2p.io/docs/
• examples/pubsub/README.md: retarget Publish/Subscribe concept link to https://libp2p.io/concepts/publish-subscribe

Repro

# broken (404)
curl -sI -L -o /dev/null -w '%{http_code}\n' https://docs.libp2p.io
curl -sI -L -o /dev/null -w '%{http_code}\n' https://docs.libp2p.io/concepts/publish-subscribe

# fixed targets (200)
curl -sI -L -o /dev/null -w '%{http_code}\n' https://libp2p.io/docs/
curl -sI -L -o /dev/null -w '%{http_code}\n' https://libp2p.io/concepts/publish-subscribe

Observed 2026-09-28 ~15:00 CST: hub and concept paths on docs.libp2p.io → 404; libp2p.io replacements → 200.

Notes

Docs-only. Does not touch Notable Users / Polygon Edge or examples/go.mod (covered separately).

[seetadev]

@imitater-dou : Wish if you could raise this PR in the repo libp2p/unified-website. Thank you so much for sharing this feedback. Will ask the sysadmin to work on it, right away.