Fix invalid links

Author: byyue

_overviews/scaladoc/contribute.md

@@ -25,4 +25,4 @@ which covers the steps and workflow necessary work on the Scaladoc tool.
 As of Scala 2.13, the Scaladoc tool is maintained but not actively
 developed.  Major development of Scaladoc will progress as a part of
 Dotty for Scala 3 in the
-[Dottydoc](https://dotty.epfl.ch/docs/usage/dottydoc.html) tool.
+[Dottydoc](https://dotty.epfl.ch/3.0.0/docs/usage/dottydoc.html) tool.

_scala3-reference/other-new-features/explicit-nulls.md

@@ -537,4 +537,4 @@ Our strategy for binary compatibility with Scala binaries that predate explicit
 and new libraries compiled without `-Yexplicit-nulls` is to leave the types unchanged
 and be compatible but unsound.
 
-[More details](https://dotty.epfl.ch/docs/internals/explicit-nulls.html)
+[More details](https://docs.scala-lang.org/scala3/reference/other-new-features/explicit-nulls.html)

scala3/scaladoc.md

@@ -81,5 +81,5 @@ If you encounter a bug or have an idea for improvement, don't hesitate to create
 
 [scaladoc-docstrings]: https://dotty.epfl.ch/docs/usage/scaladoc/docstrings.html
 [static-documentation]: https://dotty.epfl.ch/docs/usage/scaladoc/static-site.html
-[built-in-blog]: https://dotty.epfl.ch/docs/usage/scaladoc/blog.html
+[built-in-blog]: https://dotty.epfl.ch/3.0.0/docs/usage/scaladoc/blog.html
 [social-links]: https://dotty.epfl.ch/docs/usage/scaladoc/settings.html#-social-links