Add dispatch rules how-to guide to docs site

[sergeyklay]

Sortie gains rule-based dispatch in this milestone (#435): operators can map issue metadata (labels, issue type, priority) to a specific agent adapter and workflow template. The user-facing docs site needs a how-to page that walks a reader through configuring these rules from zero.

Scope of the new page: a concrete end-to-end example (e.g., route bug labels to one agent/template and docs labels to another), the full config field reference for the chosen rule format (follows the ADR from #434), matching semantics (first-match-wins vs weighted priority, glob vs exact), fallback behavior when no rule matches, and a troubleshooting section covering common misconfigurations. Cross-link to the architecture reference page (#437) and to the existing WORKFLOW.md documentation.

Depends on #434 (format ADR) and tracks #435 (implementation).

Verify: a new page under how-to/ on docs.sortie-ai.com covers the scenario above, builds cleanly, and is linked from the how-to index and the WORKFLOW.md reference page.