Broken links found by CI

[SethTisue]

seen at https://github.com/scala/scala-lang/runs/3572511446:

- ./_site/blog/2020/11/19/scala-3-forward-compat.html
  *  External link scalacenter.github.io/scala-3-migration-guide/docs/compatibility.html#the-scala-2-tasty-reader failed: 404 No error
- ./_site/community/index.html
  *  External link twitter.github.com/scala_school failed: 404 No error
- ./_site/news/2013/03/20/gsoc13.html
  *  External link colladoc.github.com failed: 404 No error
  *  External link scala-lms.github.com failed: 404 No error
- ./_site/pastevents/index.html
  *  External link archive.sphere.it/previous-editions/#2019 failed: 404 No error
  *  External link scalapeno.org.il failed: 403 No error

volunteer to fix them?

[vincenzobaz]

Why wasn't the check run on the PR?

[ashawley]

A few web sites were throttling us for checking links too often with the robot script, so it's only run as a scheduled job.