Add upgrading guide

Author: Pijukatel

docs/04_upgrading/upgrading_to_v3.md

@@ -9,6 +9,38 @@ This page summarizes the breaking changes between Apify Python SDK v2.x and v3.0
 
 Support for Python 3.9 has been dropped. The Apify Python SDK v3.x now requires Python 3.10 or later. Make sure your environment is running a compatible version before upgrading.
 
+## Actor initialization and ServiceLocator changes
+
+`Actor` initialization and global `service_locator` services setup is more strict and predictable.
+- Services in `Actor` can't be changed after calling `Actor.init`, entering the `async with Actor` context manager or after requesting them from the `Actor`
+- Services in `Actor` can be different from services in Crawler
+
+**Now (v3.0):**
+
+```python
+from crawlee.crawlers import BasicCrawler
+from crawlee.storage_clients import MemoryStorageClient, FileSystemStorageClient
+from crawlee.configuration import Configuration
+from crawlee.events import LocalEventManager
+from apify import Actor
+
+async def main():
+
+    async with Actor():
+        # This crawler will use same services as Actor and global service_locator
+        crawler_1 = BasicCrawler()
+
+        # This crawler will use custom services
+        custom_configuration = Configuration()
+        custom_event_manager = LocalEventManager.from_config(custom_configuration)
+        custom_storage_client = MemoryStorageClient()
+        crawler_2 = BasicCrawler(
+            configuration=custom_configuration,
+            event_manager=custom_event_manager,
+            storage_client=custom_storage_client,
+        )
+```
+
 ## Storages
 
 <!-- TODO -->