📝 Add docs recommending Union over Optional and migrate source examples (#4908)

* 📝 Add docs recommending Union over Optional

* 📝 Update docs recommending Union over Optional

* 📝 Update source examples for docs, recommend Union over Optional

* 📝 Update highlighted lines with updated source examples

* 📝 Update highlighted lines in Markdown with recent code changes

* 📝 Update docs, use Union instead of Optional

* ♻️ Update source examples to recommend Union over Optional

* 🎨 Update highlighted code in Markdown after moving from Optional to Union

docs_src/encoder/tutorial001.py

@@ -1,5 +1,5 @@
 from datetime import datetime
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 from fastapi.encoders import jsonable_encoder
@@ -11,7 +11,7 @@ fake_db = {}
 class Item(BaseModel):
     title: str
     timestamp: datetime
-    description: Optional[str] = None
+    description: Union[str, None] = None
 
 
 app = FastAPI()