doc: Add '--keep-going' flag to sphinx-build

So that we see all the potential errors in CI and not just the first
and can assess how much work there is to do.

This was introduced for debuggin but gmarull has suggested keeping it
in.

Signed-off-by: Michael Jones <[email protected]>

.github/workflows/doc-build.yml

@@ -85,7 +85,7 @@ jobs:
           DOC_TARGET="html"
         fi
 
-        DOC_TAG=${DOC_TAG} SPHINXOPTS="-q -W -t publish" make -C doc ${DOC_TARGET}
+        DOC_TAG=${DOC_TAG} SPHINXOPTS="-q -W --keep-going -t publish" make -C doc ${DOC_TARGET}
 
     - name: compress-docs
       run: |