model-registry: update OWNERS files (#4341)

This updates OWNERS files for directories owned by model-registry
(Kubeflow Hub Team) to include missing entries from the root OWNERS in
the model-registry repo's main branch which is currently:
https://github.com/kubeflow/model-registry/blob/ed628485a973bf4b9205e4398578dbaedee13a1b/OWNERS

Signed-off-by: Jon Burdo <jon@jonburdo.com>

Author: jonburdo

content/en/docs/components/model-registry/OWNERS

@@ -4,6 +4,13 @@ labels:
 approvers:
   - Al-Pragliola
   - ederign
+  - jonburdo
+  - pboyd
   - rareddy
   - tarilabs
+emeritus_approvers:
+  - ckadner
   - Tomcli
+  - zijianjoy
+reviewers:
+  - fege

layouts/shortcodes/model-registry/OWNERS

@@ -1,6 +1,13 @@
 approvers:
   - Al-Pragliola
   - ederign
+  - jonburdo
+  - pboyd
   - rareddy
   - tarilabs
+emeritus_approvers:
+  - ckadner
   - Tomcli
+  - zijianjoy
+reviewers:
+  - fege