linkchecker: add a reminder on broken links to add new/renamed pages to SUMMARY.md for mdBooks
          #131737
        
          
      
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I spent an embarrassingly long amount of time trying to figure out why CI was failing for a PR adding new platform support docs. In turns out it's because the PR author didn't register the new page in
SUMMARY.md. I completely forgot about it too, and was reading linkchecker source because I thought it was a bug in linkchecker.So this PR adds a note to modify
SUMMARY.mdwhen adding new pages in a mdBook.E.g.