Replace obsolete links (#7809)

Updated the obsolete link with new and maintained one.

Old link contents had been obsoleted since (02/2023) and contents are migrated to https://btrfs.readthedocs.io and https://btrfs.docs.kernel.org

New links also include new changes and features.

src/data/roadmaps/docker/content/101-underlying-technologies/102-union-filesystems.md

@@ -6,5 +6,5 @@ Visit the following resources to learn more:
 
 - [@article@AUFS (Advanced Multi-Layered Unification Filesystem)](http://aufs.sourceforge.net/)
 - [@article@OverlayFS (Overlay Filesystem)](https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html)
-- [@article@Btrfs (B-Tree Filesystem)](https://btrfs.wiki.kernel.org/index.php/Main_Page)
+- [@article@Btrfs (B-Tree Filesystem)](https://btrfs.readthedocs.io/en/stable/)
 - [@article@ZFS (Z File System)](https://zfsonlinux.org/)