Add a Developer Guide page covering the admin `?action=` JSON endpoints:
- The PSR-4 controllers under Admin\Ajax that replaced the retired ~4985-line
Views/admin/api.php (PR #173) — BaseAjaxController scaffolding
(ok/fail/gate/gateAny/requireXhr/json), LineStateTrait, per-area controllers,
route registration and the dispatchApi → AjaxController fallback order.
- The structured search JSON contract (PR #174): envelope, item shape,
self-describing actions, per-entity data, and the client-side card renderer;
plus the note that only the render path changed (matching is unchanged; a
missing live stream means a stale streams FULLTEXT index).
Wire it into the Developer Guide nav (+ ru nav_translations) and cross-link it
from HTTP Request Handling. English source only; docs/ru is regenerated before
release.
