Create an agentic workflow that automatically applies documentation labels to PRs

[shenglol]

Implement an agentic workflow that automatically identifies pull requests requiring updates to public-facing documentation (https://learn.microsoft.com/en-us/azure/azure-resource-manager/bicep/) and applies the appropriate Docs Needed or Docs Not Needed label.

Changes that require documentation updates usually include:

• New linter rules.
• New language constructs (keywords, syntax, decorators, functions, operators, etc.).
• GA of experimental features.
• New language features or capabilities, even if they don't introduce new syntax (for example, new module capabilities or deployment behaviors).
• Changes to diagnostics or compiler behavior that affect how users write or troubleshoot Bicep code.
• Changes to configuration files or settings that users are expected to author (for example, bicepconfig.json).