broken links to Dotty docs

[SethTisue]

CI is failing with:

- ./_site/overviews/scaladoc/contribute.html
[31](https://github.com/scala/docs.scala-lang/runs/5393548171?check_suite_focus=true#step:7:31)
  *  External link [dotty.epfl.ch/docs/usage/dottydoc.html](https://dotty.epfl.ch/docs/usage/dottydoc.html) failed: 404 No error
[32](https://github.com/scala/docs.scala-lang/runs/5393548171?check_suite_focus=true#step:7:32)

[33](https://github.com/scala/docs.scala-lang/runs/5393548171?check_suite_focus=true#step:7:33)
- ./_site/scala3/reference/other-new-features/explicit-nulls.html
[34](https://github.com/scala/docs.scala-lang/runs/5393548171?check_suite_focus=true#step:7:34)
  *  External link [dotty.epfl.ch/docs/internals/explicit-nulls.html](https://dotty.epfl.ch/docs/internals/explicit-nulls.html) failed: 404 No error
[35](https://github.com/scala/docs.scala-lang/runs/5393548171?check_suite_focus=true#step:7:35)
- ./_site/scala3/scaladoc.html
[36](https://github.com/scala/docs.scala-lang/runs/5393548171?check_suite_focus=true#step:7:36)

[37](https://github.com/scala/docs.scala-lang/runs/5393548171?check_suite_focus=true#step:7:37)
  *  External link [dotty.epfl.ch/docs/usage/scaladoc/blog.html](https://dotty.epfl.ch/docs/usage/scaladoc/blog.html) failed: 404 No error
  *