[material-ui][Skeleton] Document text border radius customization

[meaqua9420]

Addresses #38190

Summary

• Add a Skeleton docs note explaining how to keep text sizing behavior while changing border radius.
• Add a focused demo using variant="text" with sx={{ borderRadius: 1 }}.
• Generate the JavaScript and preview files from the TypeScript demo.

Verification

• pnpm docs:typescript:formatted
• pnpm prettier
• pnpm eslint docs/data/material/components/skeleton/SkeletonTextWithCustomRadius.tsx docs/data/material/components/skeleton/SkeletonTextWithCustomRadius.js
• git diff --cached --check

[meaqua9420]

Could a maintainer please add the appropriate labels to this PR? The test-label-applied check is failing because the PR currently has no labels; docs and scope: skeleton seem like the right fit.

I also checked the CircleCI Unable to parse YAML status: it is already present on the current master commit (ad77f4e3d76e02c59345eb964901a9be15d0dfee), and this PR only changes Skeleton docs/demo files, so that status looks unrelated to this PR diff.