QA: add yamllint workflow and config

... to check the code style of the re-usable workflows.

.github/workflows/yamllint.yml

@@ -0,0 +1,14 @@
+name: Yamllint
+
+on:
+  # Run on all pushes and on all pull requests.
+  push:
+  pull_request:
+  # Allow manually triggering the workflow.
+  workflow_dispatch:
+
+jobs:
+  yamllint:
+    uses: PHPCSStandards/.github/.github/workflows/reusable-yamllint.yml@main
+    with:
+      strict: true

.yamllint.yml

@@ -0,0 +1,21 @@
+# Details on the default config:
+# https://yamllint.readthedocs.io/en/stable/configuration.html#default-configuration
+extends: default
+
+yaml-files:
+  - '*.yaml'
+  - '*.yml'
+
+# Rule documentation: https://yamllint.readthedocs.io/en/stable/rules.html
+rules:
+  colons:
+    max-spaces-after: -1  # Disabled to allow aligning of values.
+  comments:
+    min-spaces-from-content: 1
+  comments-indentation: {}
+  document-start:
+    present: false
+  line-length:
+    max: 145
+  truthy:
+    allowed-values: ["true", "false", "on", "off"]