Update flatpak

flatpak.md

@@ -1,16 +1,19 @@
 # Related Documentation
-- [Flatpak](flatpak)
-- [snaps](snaps)
-- [Bubblewarp](bubblewrap)
-- [Firejail](firejail)
-- [landlock](landlock)
-- [systemd-nspawn](systemd-nspawn)
-- [runc](runc)
-- [crun](crun)
-- [docker](docker)
-- [dockerswarm](dockerswarm)
-- [kubernetes](kubernetes)
-- [lxc/lxd](lxd)
+- Application containers/sandboxing
+  - [Flatpak](flatpak)
+  - [snaps](snaps)
+  - [Bubblewarp](bubblewrap)
+  - [Firejail](firejail)
+  - [landlock](landlock)
+  - [systemd-nspawn](systemd-nspawn)
+  - [runc](runc)
+  - [crun](crun)
+  - [docker](docker)
+- container orchestration
+  - [dockerswarm](dockerswarm)
+  - [kubernetes](kubernetes)
+- system containers
+  - [lxc/lxd](lxd)
 - virtualized runtimes
   - [gVisor](gVisor)
   - [nabla-containers](nabla-containers)