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/nested-suspense.spec.js

@@ -17,6 +17,9 @@ test("nested suspense resolves", async ({ page }) => {
     "The robot becomes sentient and says hello world"
   );
 
+  // And expect the title to have resolved on the client
+  await expect(page).toHaveTitle("The robot says hello world");
+
   // And more loading text for the nested suspense
   await expect(mainMessageDiv).toContainText("Loading 1...");
   await expect(mainMessageDiv).toContainText("Loading 2...");