docs: add tool guardrails documentation

[TasmeerJamali]

Summary

Add comprehensive documentation for tool input and output guardrails to docs/guardrails.md. This addresses the gap identified in #1830 where tool guardrails (implemented in PR #1792) were not documented in the main guides.

The new documentation explains:

• The difference between agent-level and tool-level guardrails
• How to use @tool_input_guardrail and @tool_output_guardrail decorators
• The three behavior types: allow, reject_content, and raise_exception
• How to attach guardrails to tools and handle exceptions

Test plan

• Verified documentation builds successfully with uv run mkdocs build
• All code examples use valid Python syntax with correct imports
• Formatting is consistent with existing documentation style

Issue number

Closes #1830

Checks

• I've added new tests (if relevant) - N/A (documentation only)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass