docs: Rewrite and restructure the documentation website (#933)

Co-authored-by: aklinker1 <aklinker1@users.noreply.github.com>

docs/guide/essentials/unit-testing.md

@@ -0,0 +1,80 @@
+# Unit Testing
+
+[[toc]]
+
+## Vitest
+
+WXT provides first class support for Vitest for unit testing:
+
+```ts
+// vitest.config.ts
+import { defineConfig } from 'vitest/config';
+import { WxtVitest } from 'wxt/testing';
+
+export default defineConfig({
+  plugins: [WxtVitest()],
+});
+```
+
+This plugin does several things:
+
+- Polyfills the extension API, `browser`, with an in-memory implementation using [`@webext-core/fake-browser`](https://webext-core.aklinker1.io/fake-browser/installation)
+- Adds all vite config or plugins in `wxt.config.ts`
+- Configures auto-imports (if enabled)
+- Applies internal WXT vite plugins for things like [bundling remote code](/guide/essentials/remote-code)
+- Sets up global variables provided by WXT (`import.meta.env.BROWSER`, `import.meta.env.MANIFEST_VERSION`, `import.meta.env.IS_CHROME`, etc)
+- Configures aliases (`@/*`, `@@/*`, etc) so imports can be resolved
+
+Here are real projects with unit testing setup. Look at the code and tests to see how they're written.
+
+- [`aklinker1/github-better-line-counts`](https://github.com/aklinker1/github-better-line-counts)
+- [`wxt-dev/examples`'s Vitest Example](https://github.com/wxt-dev/examples/tree/main/examples/vitest-unit-testing)
+
+### Example Tests
+
+This example demonstrates that you don't have to mock `browser.storage` (used by `wxt/storage`) in tests - [`@webext-core/fake-browser`](https://webext-core.aklinker1.io/fake-browser/installation) implements storage in-memory so it behaves like it would in a real extension!
+
+```ts
+import { describe, it, expect } from 'vitest';
+import { fakeBrowser } from 'wxt/testing';
+
+const accountStorage = storage.defineItem<Account>('local:account');
+
+async function isLoggedIn(): Promise<Account> {
+  const value = await accountStorage.getValue();
+  return value != null;
+}
+
+describe('isLoggedIn', () => {
+  beforeEach(() => {
+    // See https://webext-core.aklinker1.io/fake-browser/reseting-state
+    fakeBrowser.reset();
+  });
+
+  it('should return true when the account exists in storage', async () => {
+    const account: Account = {
+      username: '...',
+      preferences: {
+        // ...
+      },
+    };
+    await accountStorage.setValue(account);
+
+    expect(await isLoggedIn()).toBe(true);
+  });
+
+  it('should return false when the account does not exist in storage', async () => {
+    await accountStorage.deleteValue();
+
+    expect(await isLoggedIn()).toBe(false);
+  });
+});
+```
+
+## Other Testing Frameworks
+
+To use a different framework, you will likely have to disable auto-imports, setup import aliases, manually mock the extension APIs, and setup the test environment to support all of WXT's features that you use.
+
+It is possible to do, but will require a bit more setup. Refer to Vitest's setup for an example of how to setup a test environment:
+
+https://github.com/wxt-dev/wxt/blob/main/packages/wxt/src/testing/wxt-vitest-plugin.ts