Check the YAML code in the repo with a linter

This check will comment right in the pull request on the lines
which have style violations.

Resolve a few found style violations.

Configure yamllint to skip the 'truthy' check because
otherwise it shows an error on completely fine line 4
in .github/workflows/yaml-lint.yml

See the docs in https://yamllint.readthedocs.io/en/stable/index.html

.github/workflows/yaml-lint.yml

@@ -0,0 +1,21 @@
+---
+name: Lint YAML code
+
+on:
+  push:
+    branches:
+      - '**'
+    tags-ignore:
+      - '**'
+jobs:
+  yamllint:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+
+      - name: YAML Lint and Annotate
+        uses: Staffbase/[email protected]
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          target-path: .

.yamllint.yml

@@ -0,0 +1,3 @@
+---
+rules:
+  truthy: disable

cleanup/action.yml

@@ -1,3 +1,4 @@
+---
 name: 'Clean workspace'
 description: 'Clean workspace directory after previous workflow run'
 runs: