📝 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/query_params/tutorial006.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -7,7 +7,7 @@ app = FastAPI()
 
 @app.get("/items/{item_id}")
 async def read_user_item(
-    item_id: str, needy: str, skip: int = 0, limit: Optional[int] = None
+    item_id: str, needy: str, skip: int = 0, limit: Union[int, None] = None
 ):
     item = {"item_id": item_id, "needy": needy, "skip": skip, "limit": limit}
     return item