build.yml: add doc building test for latex/pdf

.github/workflows/build.yml

@@ -73,12 +73,19 @@ jobs:
       with:
         name: stubs
         path: circuitpython-stubs*
-    - name: Docs
+    - name: Test Documentation Build (HTML)
       run: sphinx-build -E -W -b html -D version=${{ env.CP_VERSION }} -D release=${{ env.CP_VERSION }} . _build/html
     - uses: actions/upload-artifact@v2
       with:
         name: docs
         path: _build/html
+    - name: Test Documentation Build (LaTeX/PDF)
+      run: |
+        make latexpdf
+    - uses: actions/upload-artifact@v2
+      with:
+        name: docs
+        path: _build/latex
     - name: Translations
       run: make check-translate
     - name: New boards check