Add docs, example ConfigMap, describer, and review fixes

- Add Default Resource Modifiers section to restore-resource-modifiers.md
- Add --default-resource-modifier-configmap to customize-installation.md
- Add examples/default-resource-modifier-cni.yaml with CNI annotation
  stripping rules for OVN-K and Multus
- Update restore describer to show SkipDefaultResourceModifier when set
- Log warning when ResourceModifier Kind is not ConfigMap instead of
  silently doing nothing
- Add deployment_test.go coverage for the new server flag

Signed-off-by: Shubham Pampattiwar <spampatt@redhat.com>

examples/default-resource-modifier-cni.yaml

@@ -0,0 +1,18 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: default-restore-resource-modifiers
+  namespace: velero
+data:
+  resource-modifiers.yaml: |
+    version: v1
+    resourceModifierRules:
+    - conditions:
+        groupResource: pods
+      mergePatches:
+      - patchData: |
+          metadata:
+            annotations:
+              k8s.ovn.org/pod-networks: null
+              k8s.v1.cni.cncf.io/network-status: null
+              k8s.v1.cni.cncf.io/networks-status: null