Add reactions framework reference page to docs site

[sergeyklay]

Auto-merge lands as a new reaction kind in this milestone (#417). The docs site needs a reference page for the reactions framework that documents the existing ci_failure and review_comments reactions already shipped in M17, and adds auto_merge as the third entry.

Scope: explain the shared reaction model (poll -> fingerprint -> deduplicate -> dispatch -> escalate) at a level that lets operators reason about retry budgets and escalation across all reaction kinds without reading the source. Document every field of reactions.auto_merge (enabled, strategy, require_ci, delete_branch) with defaults, valid values, and safety behavior (Sortie-managed label check, draft PR exclusion). Include a full WORKFLOW.md snippet that opts in with conservative settings, and call out the irreversibility of the action. Back-fill reference entries for reactions.ci_failure and reactions.review_comments in the same page since they currently live only in release notes.

Depends on #417 (auto-merge implementation).

Verify: a new reference/reactions.md page exists on docs.sortie-ai.com, covers all three reaction kinds with field tables, and is linked from the reference index and the WORKFLOW.md reference page.