docs(streaming): add section on token usage tracking

[Pouyanpi]

Add documentation about token usage tracking when streaming is enabled in NeMo Guardrails.

PR #1264

TODO:

• text completion models like gpt-3.5-turbo-instruct do not support token tracking in streaming.

[Copilot]

Pull Request Overview

This PR adds a new section on token usage tracking when streaming is enabled in NeMo Guardrails documentation, covering usage details, version requirements, and methods to access token usage data.

• Introduces a “Token Usage Tracking” section with key features
• Specifies version requirements for LangChain packages
• Provides code examples to retrieve token usage information

Comments suppressed due to low confidence (3)

docs/user-guides/advanced/streaming.md:82

• [nitpick] Add a period at the end of this bullet for consistent punctuation in the list.

- Can be safely passed to all LLM providers since those that don't support it will simply ignore the parameter

docs/user-guides/advanced/streaming.md:80

• [nitpick] Add a period at the end of this bullet for consistent punctuation in the list.

- Provides token usage statistics even when streaming responses

docs/user-guides/advanced/streaming.md:120

• The link to generation-options.md should likely be prefixed with ../generation-options.md so it resolves correctly from this directory.

For more information about streaming token usage support across different providers, refer to the [LangChain documentation on token usage tracking](https://python.langchain.com/docs/how_to/chat_token_usage_tracking/#streaming). For detailed information about accessing generation logs and token usage, see the [Generation Options](generation-options.md#detailed-logging-information) and [Detailed Logging](../detailed-logging/README.md) documentation.

[github-actions]

Documentation preview

https://nvidia.github.io/NeMo-Guardrails/review/pr-1282

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.45%. Comparing base (0d3ddfc) to head (90acdfa).
Report is 8 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1282      +/-   ##
===========================================
+ Coverage    69.64%   70.45%   +0.80%     
===========================================
  Files          161      161              
  Lines        16055    16214     +159     
===========================================
+ Hits         11181    11423     +242     
+ Misses        4874     4791      -83     

Flag	Coverage Δ
python	70.45% <ø> (+0.80%)	⬆️

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[trebedea]

Looks good, after applying the Copilot changes.