add gcp import secrets docs (#537)

Co-authored-by: Tim Holm <tim.holm@nitric.io>

src/pages/reference/providers/gcp/configuration.mdx

@@ -28,6 +28,17 @@ gcp-project-id: my-gcp-project-id
 # https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
 schedule-timezone: Australia/Sydney # Available since v0.27.0
 
+# Import existing GCP Resources
+# Currently only secrets are supported
+# Available since v1.4.0
+import:
+  # A map of nitric names to GCP Secret Manager names
+  secrets: # Available since v1.4.0
+    # NOTE: Imported secrets must exist in the same project as specified by gcp-project-id
+    # In typescript this would import the provided secret reference for a secret declared as
+    # const mySecret = secret('my-secret');
+    my-secret: existing-secret
+
 # Configure your deployed services
 config:
   # How services without a type will be deployed