docs: add GuardrailsAI integration user guide and example

[Pouyanpi]

Description

Add documentation for integrating GuardrailsAI validators with NeMo Guardrails.

[github-actions]

Documentation preview

https://nvidia.github.io/NeMo-Guardrails/review/pr-1357

[Pouyanpi]

@zsimjee docs were missing from #1298 . Would you please take it over from here. It is better to get this merged by the end of this week.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.62%. Comparing base (9a1d178) to head (9538ba4).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1357      +/-   ##
===========================================
- Coverage    71.68%   71.62%   -0.07%     
===========================================
  Files          168      171       +3     
  Lines        16862    17020     +158     
===========================================
+ Hits         12088    12191     +103     
- Misses        4774     4829      +55     

Flag	Coverage Δ
python	71.62% <ø> (-0.07%)	⬇️

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[zsimjee]

This looks great Pouyan. Thank you