docs: Add the firestore_setup_client_create_with_project_id region tag (

GoogleCloudPlatform#8251)

The Firestore quickstart_new_instance sample can be used to demonstrate setting up client using a project id.

With this region tag, we can include it into this doc: https://cloud.google.com/firestore/docs/samples/firestore-setup-client-create-with-project-id

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

firestore/cloud-client/snippets.py

@@ -21,12 +21,14 @@
 
 def quickstart_new_instance():
     # [START firestore_setup_client_create]
+    # [START firestore_setup_client_create_with_project_id]
     from google.cloud import firestore
 
     # The `project` parameter is optional and represents which project the client
     # will act on behalf of. If not supplied, the client falls back to the default
     # project inferred from the environment.
     db = firestore.Client(project='my-project-id')
+    # [END firestore_setup_client_create_with_project_id]
     # [END firestore_setup_client_create]
 
     return db
@@ -154,7 +156,7 @@ def to_dict(self):
         # [END_EXCLUDE]
 
     def __repr__(self):
-        return(
+        return (
             f'City(\
                 name={self.name}, \
                 country={self.country}, \