GHA: add yamlcheck

Uses `yamllint`.

Also:
- fix warning for existing YAML files:
  ```
  [truthy] truthy value should be one of [false, true]
  ```

Closes #14650

.github/scripts/yamlcheck.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+# Copyright (C) Viktor Szakats
+#
+# SPDX-License-Identifier: curl
+
+set -eu
+
+# shellcheck disable=SC2046
+yamllint \
+  --format standard \
+  --strict \
+  --config-data "$(dirname "$0")/yamlcheck.yaml" \
+  $(git ls-files '*.yaml' '*.yml')

.github/scripts/yamlcheck.yaml

@@ -0,0 +1,17 @@
+# Copyright (C) Viktor Szakats
+#
+# SPDX-License-Identifier: curl
+#
+# Docs: https://yamllint.readthedocs.io/en/stable/configuration.html
+
+extends: default
+
+rules:
+  line-length:
+    max: 500
+    level: warning
+
+  braces: disable
+  commas: disable
+  comments: disable
+  document-start: disable