Try parallel Sphinx builds and hide progress

Author: wouterj

.github/workflows/ci.yaml

@@ -41,7 +41,7 @@ jobs:
                 run: pip install -r _build/.requirements.txt
 
             -   name: "Build documentation"
-                run: make -C _build SPHINXOPTS=-nW html
+                run: make -C _build SPHINXOPTS="-nqW -j2" html
 
     doctor-rst:
         name: DOCtor-RST