📝 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/additional_responses/tutorial004.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 from fastapi.responses import FileResponse
@@ -25,7 +25,7 @@ app = FastAPI()
     response_model=Item,
     responses={**responses, 200: {"content": {"image/png": {}}}},
 )
-async def read_item(item_id: str, img: Optional[bool] = None):
+async def read_item(item_id: str, img: Union[bool, None] = None):
     if img:
         return FileResponse("image.png", media_type="image/png")
     else: