ci: start signing .drone.yml

* also ignore .drone.yml for pre-commit yaml check, as it is already
  being checked by 'drone lint' and multi-doc yaml files make the check
  troubled.

.drone.yml

@@ -66,4 +66,9 @@ steps:
 
 trigger:
   event: [push, pull_request, cron, tag]
+
+---
+kind: signature
+hmac: 4332a4549f9f31d144d8d663527c62b216d536320d45049a9d5790e67076dc1c
+
 ...

.pre-commit-config.yaml

@@ -6,6 +6,7 @@ repos:
     hooks:
       - id: check-merge-conflict
       - id: check-yaml
+        exclude: ^.drone.yml$
   - repo: local
     hooks:
       # this hook requires that the drone-cli tool is installed