📝 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/tutorial003.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI
 
@@ -6,7 +6,7 @@ app = FastAPI()
 
 
 @app.get("/items/{item_id}")
-async def read_item(item_id: str, q: Optional[str] = None, short: bool = False):
+async def read_item(item_id: str, q: Union[str, None] = None, short: bool = False):
     item = {"item_id": item_id}
     if q:
         item.update({"q": q})