auto-update MathCAT inside of MathCATDemo

When there's a change in [MathCAT](https://github.com/daisy/MathCAT), e.g. an extension/fix in the rules of a language, how does this get reflected in https://daisy.github.io/MathCATDemo/?
This website auto-deploys when there's a commit in the `main` branch in this repo, right? So that is totally unrelated to what happens in [MathCAT](https://github.com/daisy/MathCAT) ? Is this a good way to handle it? Does it make sense to have weekly re-deploy maybe? Not having a strong opinion here, just collecting some first ideas.

Start by inspecting how MathCATDemo deploys from the main branch and how it obtains MathCAT, then review the linked MathCAT repository's change flow. Define whether MathCAT changes should trigger or schedule deployments, and consider the work complete when an agreed update path reliably reflects MathCAT changes in the live demo.

Written by the indexing model from the issue text.