Update git submodules

* Update docs/submodules/apex from branch 'stable/danube'
  - Updates docs for Danube 3.0

    Change-Id: I586c188b8cbf5d8a0c47926d6a816652515495ac
    Signed-off-by: Tim Rozet <trozet@redhat.com>

diff --git a/docs/submodules/apex b/docs/submodules/apex
index ec255e7..f4f6eea 160000 (submodule)
--- a/docs/submodules/apex
+++ b/docs/submodules/apex
@@ -1 +1 @@
-Subproject commit ec255e70792340c86d5e1a975ac1ae94b135fb8e
+Subproject commit f4f6eeab383e808caa4756fc8c31fe684983b234