Stop using <h1> in example README.md

[abey79]

What

<h1> is for the page title, that is autogenerated by the front end. The README should only contains <h2> and lower. This PR fixes all the readme and adds a lint for that (available with python scripts/lint.py --extra).

Would benefit from https://github.com/rerun-io/landing/issues/521 being solved.

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: rerun.io/viewer
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

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