docs: fill changelog version link refs (0.4.0/0.3.0/0.2.0)

CHANGELOG.md

@@ -265,4 +265,7 @@ alknet mono-repo.
   the client→backend pump is aborted at session end instead of
   lingering until the client disconnects.
 
+[0.4.0]: https://git.alk.dev/alkdev/alktty/releases/tag/v0.4.0
+[0.3.0]: https://git.alk.dev/alkdev/alktty/releases/tag/v0.3.0
+[0.2.0]: https://git.alk.dev/alkdev/alktty/releases/tag/v0.2.0
 [0.1.0]: https://git.alk.dev/alkdev/alktty/releases/tag/v0.1.0