Add yamllint (#26965)

So that https://github.com/go-gitea/gitea/pull/26964 does not happen
again. Merge this after that PR. Config is based on
[node's](https://github.com/nodejs/node/blob/main/.yamllint.yaml).

---------

Co-authored-by: techknowlogick <techknowlogick@gitea.com>

.github/stale.yml

@@ -9,8 +9,8 @@ daysUntilClose: 14
 
 # Issues or Pull Requests with these labels will never be considered stale. Set to `[]` to disable
 exemptLabels:
-  - status/blocked 
-  - kind/security 
+  - status/blocked
+  - kind/security
   - lgtm/done
   - reviewed/confirmed
   - priority/critical
@@ -27,7 +27,7 @@ staleLabel: stale
 
 # Comment to post when marking as stale. Set to `false` to disable
 markComment: >
-  This issue has been automatically marked as stale because it has not had recent activity. 
+  This issue has been automatically marked as stale because it has not had recent activity.
   I am here to help clear issues left open even if solved or waiting for more insight.
   This issue will be closed if no further activity occurs during the next 2 weeks.
   If the issue is still valid just add a comment to keep it alive.