Fix ReSpec build and Pages publishing

[korrayt]

Summary

• Use the default export exposed by mathml-polyfills so the ReSpec build can load _MathTransforms.
• Keep the nested-math presentation example as source-only markup instead of wrapping it in another generated outer math element.
• Let w3c/spec-prod manage the gh-pages branch after a successful build, and correct the W3C decision URL typo.

Root cause

The latest CI build imports _MathTransforms as a named export, while all-polyfills.js exposes it as the default export. The showcore post-processing hook also wraps MathML examples in an outer math element; that made the nested-math example produce invalid generated markup. The workflow reset gh-pages before the build, so a failed build could leave the Pages branch without the generated document.

Validation

• ReSpec 37.2.0 local build completed successfully (exit code 0).
• Generated HTML contained no nested real math elements.
• Workflow YAML parsed successfully with Prettier's YAML parser.
• git diff --check passed.

Related to #522.

[davidcarlisle]

Currently all development is in main branch but we need to put the built documents on to gh-pages
If I'm reading this diff correctly you are making gh-pages the source branch? I'm not sure we want to change that at this time (although I can believe this step has issues, it has basically been working for some years?)
or are you saying spec-prod can handle that on its own now, I should check....