📝 Use Optional in docs (#1644)

* Updated .py files with Optional tag (up to body_nested_models)

* Update optionals

* docs_src/ all updates, few I was unsure of

* Updated markdown files with Optional param

* es: Add Optional typing to index.md

* Last of markdown files updated with Optional param

* Update highlight lines

* it: Add Optional typings

* README.md: Update with Optional typings

* Update more highlight increments

* Update highlights

* schema-extra-example.md: Update highlights

* updating highlighting on website to reflect .py changes

* Update highlighting for query-params & response-directly

* Address PR comments

* Get rid of unnecessary comment

* ⏪ Revert Optional in Chinese docs as it probably also requires changes in text

* 🎨 Apply format

* ⏪ Revert modified example

* ♻️ Simplify example in docs

* 📝 Update OpenAPI callback example to use Optional

* ✨ Add Optional types to tests

* 📝 Update docs about query params, default to using Optional

* 🎨 Update code examples line highlighting

* 📝 Update nested models docs to use "type parameters" instead of "subtypes"

* 📝 Add notes about FastAPI usage of None

including:

= None

and

= Query(None)

and clarify relationship with Optional[str]

* 📝 Add note about response_model_by_alias

* ♻️ Simplify query param list example

* 🔥 Remove test for removed example

* ✅ Update test for updated example

Co-authored-by: Christopher Nguyen <chrisngyn99@gmail.com>
Co-authored-by: yk396 <yk396@cornell.edu>
Co-authored-by: Kai Chen <kaichen120@gmail.com>

docs_src/schema_extra_example/tutorial002.py

@@ -1,3 +1,5 @@
+from typing import Optional
+
 from fastapi import FastAPI
 from pydantic import BaseModel, Field
 
@@ -6,9 +8,9 @@ app = FastAPI()
 
 class Item(BaseModel):
     name: str = Field(..., example="Foo")
-    description: str = Field(None, example="A very nice Item")
+    description: Optional[str] = Field(None, example="A very nice Item")
     price: float = Field(..., example=35.4)
-    tax: float = Field(None, example=3.2)
+    tax: Optional[float] = Field(None, example=3.2)
 
 
 @app.put("/items/{item_id}")