Documentation previews are broken due to lack of MathJax javascript

[kwankyu]

#36098 broke the documentation previews for PRs.

CAUSE

The docbuild is zipping up a subtree:

cp -r -L /sage/local/share/doc/sage/html/en/* ./docs
 # Zip everything for increased performance
zip -r docs.zip docs

and then publishing that somewhere. After #36098, tex-chtml.js isn't duplicated inside each _static folder,a and hence it's missing from the zip. This results in the documentation previews lacking mathjax.

SOLUTION

diff --git a/.github/workflows/doc-build.yml b/.github/workflows/doc-build.yml
index 3635980d15..eb4f81f303 100644
--- a/.github/workflows/doc-build.yml
+++ b/.github/workflows/doc-build.yml
@@ -82,6 +82,8 @@ jobs:
         # incremental docbuild may introduce broken links (inter-file references) though build succeeds
         run: |
           set -ex
+          export SAGE_USE_CDNS=yes       
           mv /sage/local/share/doc/sage/html/en/.git /sage/.git-doc
           make doc-clean doc-uninstall sagelib-clean && git clean -fx src/sage
           mkdir -p /sage/local/share/doc/sage/html/en/ && mv /sage/.git-doc /sage/local/share/doc/sage/html/en/.git

would suffice.