📝 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/header_params/tutorial002.py

@@ -1,4 +1,4 @@
-from typing import Optional
+from typing import Union
 
 from fastapi import FastAPI, Header
 
@@ -7,6 +7,6 @@ app = FastAPI()
 
 @app.get("/items/")
 async def read_items(
-    strange_header: Optional[str] = Header(default=None, convert_underscores=False)
+    strange_header: Union[str, None] = Header(default=None, convert_underscores=False)
 ):
     return {"strange_header": strange_header}