docs: link Beacon as the v2 learning path

[codeforester]

Summary

• make Beacon a first-class, pinned learning path from the README, docs index, and v2 quickstart
• link the current demo release v0.1.2 tutorial, rationale, and adoption guide
• refresh the consumer validation ledger after the demo v0.1.2 release
• add docs-contract assertions for pinned links and learning-path wording

Validation

• git diff --check
• ./tests/docs-contract.sh

Fixes #568