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>

poetry.lock

@@ -1,4 +1,4 @@
-# This file is automatically @generated by Poetry 1.5.1 and should not be changed by hand.
+# This file is automatically @generated by Poetry 1.6.1 and should not be changed by hand.
 
 [[package]]
 name = "click"
@@ -325,7 +325,25 @@ notebook = ["ipywidgets (>=6)"]
 slack = ["slack-sdk"]
 telegram = ["requests"]
 
+[[package]]
+name = "yamllint"
+version = "1.32.0"
+description = "A linter for YAML files."
+optional = false
+python-versions = ">=3.7"
+files = [
+    {file = "yamllint-1.32.0-py3-none-any.whl", hash = "sha256:d97a66e48da820829d96077d76b8dfbe6c6140f106e558dae87e81ac4e6b30b7"},
+    {file = "yamllint-1.32.0.tar.gz", hash = "sha256:d01dde008c65de5b235188ab3110bebc59d18e5c65fc8a58267cd211cd9df34a"},
+]
+
+[package.dependencies]
+pathspec = ">=0.5.3"
+pyyaml = "*"
+
+[package.extras]
+dev = ["doc8", "flake8", "flake8-import-order", "rstcheck[sphinx]", "sphinx"]
+
 [metadata]
 lock-version = "2.0"
 python-versions = "^3.8"
-content-hash = "1b154f70c35b75d47c843959af9df0e7343f3bb579835825ca889ec9350afc41"
+content-hash = "4017ac7637dcc0703b37f8dc640d3e11e67e60c7b14660838135cff38ad91656"