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/web.spec.js

@@ -7,6 +7,8 @@ test("button click", async ({ page }) => {
   // Expect the page to contain the counter text.
   const main = page.locator("#main");
   await expect(main).toContainText("hello axum! 0");
+  // Expect the title to contain the counter text.
+  await expect(page).toHaveTitle("hello axum! 0");
 
   // Click the increment button.
   let button = page.locator("button.increment-button");
@@ -14,6 +16,8 @@ test("button click", async ({ page }) => {
 
   // Expect the page to contain the updated counter text.
   await expect(main).toContainText("hello axum! 1");
+  // Expect the title to contain the updated counter text.
+  await expect(page).toHaveTitle("hello axum! 1");
 });
 
 test("svg", async ({ page }) => {