ENH: Make the documentation build be permissive with warnings

Make the documentation build be permissive with warnings: remove the `W`
flag to avoid treating warnings as errors.

Warnings will still be reported in the GHA workflow build log.

.github/workflows/doc_build.yaml

@@ -28,4 +28,4 @@ jobs:
 
     - name: Build docs
       run: |
-          make -C doc/ SPHINXOPTS="-W -v" BUILDDIR="$HOME/docs" OUTDIR="$HOME/html" html
+          make -C doc/ SPHINXOPTS="-v" BUILDDIR="$HOME/docs" OUTDIR="$HOME/html" html