ci(docs): replace apt-get nodejs/npm with actions/setup-node@v4

apt-get pulled ~250 Ubuntu node-* packages (~20+ min, flaky-network on
sporel-meta runs #206 cancelled, #209 took >15min just downloading).
Switch to actions/setup-node@v4 — single tarball, ~30s.

Note: sporel-engine pins this workflow to a specific SHA so it's
unaffected; sporel-meta uses @master and benefits immediately.

.gitea/workflows/reusable-docs.yml

@@ -21,11 +21,13 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Install Node + markdownlint-cli2
-        run: |
-          sudo apt-get update -qq
-          sudo apt-get install -y nodejs npm
-          sudo npm install -g markdownlint-cli2
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+
+      - name: Install markdownlint-cli2
+        run: npm install -g markdownlint-cli2
 
       - name: Run markdownlint
         run: |