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>

.yamllint.yaml

@@ -0,0 +1,48 @@
+extends: default
+
+rules:
+  braces:
+    min-spaces-inside: 0
+    max-spaces-inside: 1
+    min-spaces-inside-empty: 0
+    max-spaces-inside-empty: 0
+
+  brackets:
+    min-spaces-inside: 0
+    max-spaces-inside: 1
+    min-spaces-inside-empty: 0
+    max-spaces-inside-empty: 0
+
+  comments:
+    require-starting-space: true
+    ignore-shebangs: true
+    min-spaces-from-content: 1
+
+  comments-indentation:
+    level: error
+
+  document-start:
+    level: error
+    present: false
+    ignore: |
+      /.drone.yml
+
+  document-end:
+    present: false
+
+  empty-lines:
+    max: 1
+
+  indentation:
+    spaces: 2
+
+  line-length: disable
+
+  truthy:
+    allowed-values: ["true", "false", "on", "off"]
+
+ignore: |
+  .venv
+  node_modules
+  /models/fixtures
+  /models/migrations/fixtures