Add auto collect schemas for utoipa-axum

[juhaku]

Add support for auto collecting schemas from handlers annotated with #[utoipa::path(...)] macro used with utoipa-axum routes! macro.

Update examples and docs.

From now on the there is no need to merge schemas from separate OpenApi since the schemas will get collected from the handlers.

 #[derive(utoipa::ToSchema)]
 struct User {
     id: i32,
 }

 #[utoipa::path(get, path = "/user", responses((status = OK, body = User)))]
 async fn get_user() -> Json<User> {
     Json(User { id: 1 })
 }

 let (router, api) = OpenApiRouter::new()
     .routes(routes!(get_user))
     .split_for_parts();

Closes #1065