Add content linter to CI (#42192)

Co-authored-by: Peter Bengtsson <peterbe@github.com>

.github/workflows/content-lint-markdown.yml

@@ -0,0 +1,38 @@
+name: 'Content Lint Markdown'
+
+# **What it does**: Lints our content markdown to ensure the content matches the specified styleguide.
+# **Why we have it**: We want some level of consistency to our content markdown files.
+# **Who does it impact**: Docs content writers.
+
+on:
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  lint-content:
+    if: github.repository == 'github/docs-internal' || github.repository == 'github/docs'
+    runs-on: ${{ fromJSON('["ubuntu-latest", "ubuntu-20.04-xl"]')[github.repository == 'github/docs-internal'] }}
+    steps:
+      - name: Check out repo
+        uses: actions/checkout@3df4ab11eba7bda6032a0b82a6bb43b11571feac # v4.0.0
+        with:
+          # Needed to detect changed files
+          fetch-depth: 2
+
+      - name: Set up Node and dependencies
+        uses: ./.github/actions/node-npm-setup
+
+      - name: Get changed content/data files
+        id: changed-files
+        uses: tj-actions/changed-files@246636f5fa148b5ad8e65ca4c57b18af3123e5f6 # v39.0.1
+        with:
+          fetch_depth: 2
+          files: |
+            content/**
+            data/**
+
+      - name: Run content linter if changed content/data files
+        if: steps.changed-files.outputs.any_changed == 'true'
+        run: node src/content-linter/scripts/markdownlint.js --errors-only --paths ${{ steps.changed-files.outputs.all_changed_files }}