Add elixir to docs workflow

.github/workflows/doc.yml

@@ -9,11 +9,21 @@ jobs:
     concurrency: ci-${{ github.ref }} # Recommended if you intend to make multiple deployments in quick succession.
     runs-on: ubuntu-latest
     steps:
-      - name: Checkout 🛎️
-        uses: actions/checkout@v3
-
+      - uses: actions/checkout@v3
+      - name: Set up Elixir
+        uses: erlef/setup-beam@988e02bfe678367a02564f65ca2e37726dc0268f
+        with:
+          elixir-version: '1.13.2'
+          otp-version: '24.1'
+      - name: Restore dependencies cache
+        uses: actions/cache@v3
+        with:
+          path: deps
+          key: ${{ runner.os }}-mix-${{ hashFiles('**/mix.lock') }}
+          restore-keys: ${{ runner.os }}-mix-
       - name: Install dependencies
         run: mix deps.get
+
       - name: Build doc
         run: mix docs