[docs-infra] Fix danger URL docs preview based on git diff

[oliviertassinari]

Steps to reproduce

Steps:

1. Open this link to live example: (required) [material-ui][useMediaQuery] Add warning for using useMediaQuery('print') material-ui#44790 (comment)

Current behavior

• Sometimes it works, sometimes it doesn't.
• It only kind of works on https://github.com/mui/material-ui

Expected behavior

• Simple logic
• Works in all repositories, no duplication
• It's reliable

Context

• Existing logic https://github.com/mui/material-ui/blob/9574765d827b72545a10806db1bb1f7b3d2f54b9/dangerFileContent.ts#L190
• [docs] Link to docs on PRs toolpad#4349 (review)

Your environment

npx @mui/envinfo

  Don't forget to mention which browser you used.
  Output from `npx @mui/envinfo` goes here.

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