switch to a Document trait and introduce Script/Head/Style/Meta components (#2635)

* switch to a Document trait and introduce Script/Head/Style/Meta components

* Fix desktop title

* Insert special elements into the head during the inital SSR render

* Make all head component attributes optional

* hydrate head elements

* improve the server streaming head warning

* Document fullstack head hydration approach

* deduplicate head elements by href

* move Link into head::Link

* document head components

* add meta and title examples

* Fix a few doc examples

* fix formatting

* Add title to playwright tests

* serde is optional on web, but it is enabled by hydrate

* remove leftover console log

packages/playwright-tests/fullstack/src/main.rs

@@ -17,6 +17,7 @@ fn app() -> Element {
 
     rsx! {
         h1 { "hello axum! {count}" }
+        Title { "hello axum! {count}" }
         button { class: "increment-button", onclick: move |_| count += 1, "Increment" }
         button {
             class: "server-button",