feat(aio): add API endpoint for notifying about PR updates

This commit adds an API endpoint for notifying the preview server about PR
updates (`/pr-updated`). According to the update, the preview server can take
several actions. Currently, it will only check and (if necessary) update the
PR's preview visibility (but more actions could be supported in the future).
The API can be used with an automatic trigger (e.g. a GitHub webhook) to
instantly update a PR's preview visibility when it changes.

Fixes #16526

aio/aio-builds-setup/docs/overview--http-status-codes.md

@@ -53,6 +53,28 @@ with a bried explanation of what they mean:
   Payload larger than size specified in `AIO_UPLOAD_MAX_SIZE`.
 
 
+## `https://ngbuilds.io/health-check`
+
+- **200 (OK)**:
+  The server is healthy (i.e. up and running and processing requests).
+
+
+## `https://ngbuilds.io/pr-updated`
+
+- **200 (OK)**:
+  Request processed successfully. Processing may or may not have resulted in further actions.
+
+- **400 (Bad Request)**:
+  No payload or no `number` field in payload.
+
+- **405 (Method Not Allowed)**:
+  Request method other than POST.
+
+- **409 (Conflict)**:
+  Request to overwrite existing directory (i.e. directories for both visibilities exist).
+  (Normally, this should not happen.)
+
+
 ## `https://*.ngbuilds.io/*`
 
 - **404 (Not Found)**: