feat(gandalf): declare the not-found page in bunny.toml

How a site answers missing paths now belongs to its repository like its
edge rules: bunny.toml declares [site] not_found as "default", "spa" or
a page from the build such as "/404.html". The action reads and sends
the table on start and finalize; the gateway validates it before the
upload and applies it to the Storage Zone at finalization, recording
and purging only when it changed. A deployment without [site] resets
the default; older action versions leave the setting alone.

The console's Settings tab only shows the applied value, and the admin
API no longer sets it. The maintainer guide and llms.txt describe the
setting under Configuration.

action.yml

@@ -19,9 +19,10 @@ inputs:
     default: ""
   edge-rules:
     description: >-
-      Optional TOML file with [[edge_rules]] (redirects, status codes, headers,
-      cache times) for the site's Pull Zones. Needs purge-endpoint. Through the
-      gateway, leaving it empty removes rules from earlier deployments.
+      Optional bunny.toml with the site's [[edge_rules]] (redirects, status
+      codes, headers, cache times) and [site] settings (not_found). Needs
+      purge-endpoint. Through the gateway, leaving it empty removes rules and
+      resets settings that earlier deployments declared.
     required: false
     default: ""
   dry-run: