chore: prepare for release v0.16.0

[github-actions]

🚀 Release v0.16.0

This PR was automatically created by the release workflow.

Changes included:

• ✅ Updated version to v0.16.0
• ✅ Updated CHANGELOG.md with latest changes
• ✅ Updated version references in README.md

---

After merging this PR, a tag will be created and a GitHub release will be published.

[tgasser-nv]

Pre-Release Checklist

• All tests passing on develop branch: Tested locally, these will re-run after this PR is merged to make the release.
• No outstanding critical issues: No. Removed non-critical Github Issues from the milestonf.
  [N/A] Dependencies are up to date: Not run for v0.16.0, will start doing this in v0.17.0.
• No high-severity security vulnerabilities: None reported by Dependabot.
• No pending PRs in the release milestone: No

[github-actions]

Documentation preview

https://nvidia.github.io/NeMo-Guardrails/review/pr-1362

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.62%. Comparing base (877785a) to head (a059be3).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1362   +/-   ##
========================================
  Coverage    71.62%   71.62%           
========================================
  Files          171      171           
  Lines        17021    17021           
========================================
  Hits         12192    12192           
  Misses        4829     4829           

Flag	Coverage Δ
python	71.62% <ø> (ø)

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