docs(guardrails): expand guardrails page with all validators and examples [AL-290]

[apetraru-uipath]

What changed?

Rewrote packages/uipath/docs/core/guardrails.md from a single-line mkdocstrings stub into a comprehensive reference page.

• Added prose introduction and quick-start usage example for the @guardrail decorator
• Added execution stages table (PRE / POST / PRE_AND_POST) with supported-validator matrix
• Added dedicated sections for all five built-in (Azure Content Safety-backed) validators with code examples:
  • PII Detection (PIIValidator) — 18 entity types, per-entity confidence thresholds
  • Harmful Content (HarmfulContentValidator) — 4 categories, severity thresholds 0–6
  • Prompt Injection (PromptInjectionValidator) — PRE stage only
  • User Prompt Attacks (UserPromptAttacksValidator) — PRE stage only, no params
  • Intellectual Property (IntellectualPropertyValidator) — POST stage only
• Added Actions section (LogAction, BlockAction, custom GuardrailAction)
• Added Custom Validators section (CustomValidator with 1-arg and 2-arg rule functions)
• Added GuardrailExclude usage and stacking guardrails examples
• Added Low-level API section for direct sdk.guardrails.evaluate_guardrail() usage
• Added full API Reference section with 13 targeted mkdocstrings directives covering all public classes

How has this been tested?

Verified locally with uv run mkdocs serve — all sections render correctly, all mkdocstrings references resolve without warnings.

Are there any breaking changes?

• Under Feature Flag
• None
• DB migrations required
• API removals / deprecations
• Other

[apetraru-uipath]

How to view locally the docs:

cd /uipath-python/packages/uipath
uv run mkdocs serve

Then open http://127.0.0.1:8000/uipath-python/core/guardrails/ in your browser.