Consolidate pages render check into Katex check

https://github.com/Qiskit/documentation/pull/3408 adds a new check for long math expressions. It runs over all changed files. It's similar to our pages render check in running over changed files and using ./start for a live preview of the app.

In CI, it's inefficient to run both checks because the Katex check is more powerful and could replace the pages render check. We could ensure that Playwright is checking the page 200-ed, i.e. loaded correctly. If it didn't load correctly, our Katex check should have a helpful error message.

We maybe want to keep the pages render check in general because it can be helpful to run locally and is presumably faster than our Playwright check. Although we could decide to remove it.

Inspect the existing pages render check and Katex check in the CI configuration, then review how each runs over changed files and uses ./start. Determine whether the Playwright check can replace the pages render check while reporting page-load failures clearly. Done means the CI checks are consolidated without losing the useful local pages render workflow.