Add yamllint workflow

.github/workflows/yamllint.yaml

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+name: yamllint
+
+permissions:
+  contents: read
+
+on:
+  pull_request:
+    types: [ opened, edited, reopened, synchronize, ready_for_review ]
+
+jobs:
+  yamllint-check:
+    name: yaml-lint
+    uses: metal3-io/project-infra/.github/workflows/yamllint.yaml@main
+    with:
+      ref: ${{ github.event.pull_request.head.sha }}

.yamllint.yaml

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+yaml-files:
+- '*.yaml'
+- '*.yml'
+- '.yamllint'
+
+rules:
+  trailing-spaces: enable
+  key-duplicates: enable
+  indentation:
+    spaces: 2
+    indent-sequences: false # Enforce k8s-style indentation
+    check-multi-line-strings: false
+  truthy:
+    allowed-values: [ 'true', 'false', 'yes', 'no', 'on' ]