docs: document input guardrail execution modes

[RVV-karma]

Summary

Adds documentation for the run_in_parallel parameter introduced in #1986, explaining the two execution modes for input guardrails and their use cases.

Documentation Changes:

• Updated introduction to clarify guardrails can run in parallel or blocking mode
• Added "Execution modes" section explaining:
  • Parallel execution (default) for low latency
  • Blocking execution for cost efficiency and preventing race conditions
• Added comparison table of both modes
• Added code examples demonstrating both execution modes
• Updated recommendations for when to use each mode

Context:

This documentation was split from #1986 per reviewer feedback to be merged after the feature is released in version 0.5.x.

Related PRs/Issues

• Related to feat: add run_in_parallel parameter to input guardrails #1986 (feature implementation)
• Related to Add blocking execution mode for input guardrails #1985 (original feature request)

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Checklist:

• Documentation updated
• Examples included
• Clear and accurate

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Short and focused on just the docs changes! This makes it clear it's a follow-up documentation PR. 📝