Link the released Amber v2 documentation

[crimson-knight]

Summary

• Point the v2-dev README at the published Amber v2 beta documentation.
• Keep the stable Amber 1.4.1 documentation explicitly available.
• Extend the beta contract check so the retired docs host cannot return to release-facing files.

Validation

• CRYSTAL_CACHE_DIR=/tmp/amber_beta_framework_cache bash scripts/check_beta_contract.sh
• git diff --check

Release impact

This makes the repository landing page consistent with the already published 2.0.0-beta.2 documentation and support matrix.