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).

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

.github/labeler.yml

@@ -22,3 +22,4 @@ kind/lint:
   - ".markdownlint.yaml"
   - ".spectral.yaml"
   - ".stylelintrc.yaml"
+  - ".yamllint.yaml"