fix(docs): add missing imports in documentation code snippets

docs/guides/cookbook/project_groups/get_a_project_group_by_id.md

@@ -10,6 +10,7 @@ This is what the `pyticktick` code would look like:
     import json
     from pyticktick import Client
 
+    client = Client()
     resp = client.get_batch_v2()
     if resp.project_groups is None:
         print("No project groups found.")

docs/guides/cookbook/project_groups/get_a_project_group_by_name.md

@@ -10,6 +10,7 @@ This is what the `pyticktick` code would look like:
     import json
     from pyticktick import Client
 
+    client = Client()
     resp = client.get_batch_v2()
     if resp.project_groups is None:
         print("No project groups found.")

docs/guides/ticktick_api/generate_v1_token.md

@@ -77,6 +77,8 @@ Then follow any additional prompts to complete the process.
 The actual code required to generate a V1 token is quite simple:
 
 ```python
+from time import time
+
 from pyticktick import Settings
 
 client_id = "YOUR_CLIENT_ID"