linkchecker: add a reminder on broken links to add new/renamed pages to SUMMARY.md for mdBooks

[jieyouxu]

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.md when adding new pages in a mdBook.

E.g.

# Adding a new `meow` target but forgor to register the page in `SUMMARY.md`
rustc\platform-support.html:183: broken link - `rustc\platform-support\meow.html`
rustc\print.html:9730: broken link - `rustc\platform-support\meow.html`
checked links in: 19.1s
number of HTML files scanned: 43588
number of HTML redirects found: 13735
number of links checked: 3145951
number of links ignored due to external: 156244
number of links ignored due to exceptions: 9
number of intra doc links ignored: 8
errors found: 2
NOTE: if you are adding or renaming a markdown file in a mdBook, don't forget to register the page in SUMMARY.md
found some broken links

[rustbot]

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[ehuss]

Thanks!

@bors r+ rollup

[bors]

📌 Commit 5f74fde has been approved by ehuss

It is now in the queue for this repository.