please expose update-sri file to cdn capability

I'm using katex 0.12 in node for static rendering.

Doing so includes:
- install katex npm package
- import and use `renderToString`
- in order to use katex css CDN for production I need to *copy-paste* the correct `link` from the documentation. (the one generated by `update-sri`)

It would be nice if I could get the link from the npm katex package so that there will be less chance of breakage when updating katext version (no copy-pasting, just include something like `katex.cdnMetadata.minCssUrl` and `katex.cdnMetadata.minCssIntegrity` in my server-side template)