crane/gcrane: add pages (#13853)

pages/common/crane-flatten.md

@@ -0,0 +1,17 @@
+# crane flatten
+
+> Flatten an image's layers into a single layer.
+> Pushes digest to original image repository if no tags are specified.
+> More information: <https://github.com/google/go-containerregistry/blob/main/cmd/crane/doc/crane_digest.md>.
+
+- Flatten an image:
+
+`crane flatten`
+
+- Apply new tag to flattened image:
+
+`crane flatten {{-t|--tag}} {{tag_name}}`
+
+- Display help:
+
+`crane flatten {{-h|--help}}`