Consider adding a check for git merge remnants

[parlough]

Describe the problem

Recently in https://github.com/dart-lang/site-www/commits/main, remnants of a broken git merge/rebase were accidentally included with the other changes, along with some broken text. While usually this usually will be avoided or caught, it's not great to let those breakages in by accident, causing confusing or broken text.

Expected fix

Add a CI check for common remnants of a broken or failed merged.

Additional context

No response