docs: release notes for 0.15 and more doc fixes

[miyoungc]

Description

To fill up the release notes for SDK 0.15 and improve the new content

Related Issue(s)

#1287

Checklist

• I've read the CONTRIBUTING guidelines.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• @mentions of the person or team responsible for reviewing proposed changes.

[github-actions]

Documentation preview

https://nvidia.github.io/NeMo-Guardrails/review/pr-1315

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.52%. Comparing base (dd1ce5f) to head (a1c8339).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1315   +/-   ##
========================================
  Coverage    70.52%   70.52%           
========================================
  Files          161      161           
  Lines        16267    16267           
========================================
  Hits         11473    11473           
  Misses        4794     4794           

Flag	Coverage Δ
python	70.52% <ø> (ø)

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[Pouyanpi]

@miyoungc you can see changelog updates in #1319

[miyoungc]

@miyoungc you can see changelog updates in #1319

Made some updates according to the change log @Pouyanpi

[Pouyanpi]

Thank you @miyoungc. Please open another PR for doc fixes and keep this PR for release notes and release prep. Thanks! I'll get the doc fixes PR merged first and include them in the changelog.

[miyoungc]

Thank you @miyoungc. Please open another PR for doc fixes and keep this PR for release notes and release prep. Thanks! I'll get the doc fixes PR merged first and include them in the changelog.

I can't split the changes. The doc build will break if I do so. All the doc changes are related to the release note to properly cross-reference to the right parts linking from the release notes. And plus only a few minor style fixes.

[tgasser-nv]

LGTM thanks Miyoung