Docs for DOM testing and FileSink (#3330)

* Update websocket docs & jsdoc

* Add info about user-specific data in ws

* Document FileSink

* Docs for happydom test

* Updates

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+Bun's test runner plays well with existing component and DOM testing libraries, including React Testing Library and [`happy-dom`](https://github.com/capricorn86/happy-dom).
+
+## `happy-dom`
+
+For writing headless tests for your frontend code and components, we recommend [`happy-dom`](https://github.com/capricorn86/happy-dom). Happy DOM implements a complete set of HTML and DOM APIs in plain JavaScript, making it possible to simulate a browser environment with high fidelity.
+
+To get started install the `@happy-dom/global-registrator` package as a dev dependency.
+
+```bash
+$ bun add -d @happy-dom/global-registrator
+```
+
+We'll be using Bun's _preload_ functionality to register the `happy-dom` globals before running our tests. This step will make browser APIs like `document` available in the global scope. Create a file called `happydom.ts` in the root of your project and add the following code:
+
+```ts
+import { GlobalRegistrator } from "@happy-dom/global-registrator";
+
+GlobalRegistrator.register();
+```
+
+To preload this file before `bun test`, open or create a `bunfig.toml` file and add the following lines.
+
+```toml
+[test]
+preload = "./happydom.ts"
+```
+
+This will execute `happydom.ts` when you run `bun test`. Now you can write tests that use browser APIs like `document` and `window`.
+
+```ts#dom.test.ts
+import {test, expect} from 'bun:test';
+
+test('dom test', () => {
+  document.body.innerHTML = `<button>My button</button>`;
+  const button = document.querySelector('button');
+  expect(button?.innerText).toEqual('My button');
+});
+```
+
+Depending on your `tsconfig.json` setup, you may see a `"Cannot find name 'document'"` type error in the code above. To "inject" the types for `document` and other browser APIs, add the following [triple-slash directive](https://www.typescriptlang.org/docs/handbook/triple-slash-directives.html) to the top of any test file.
+
+```ts-diff#dom.test.ts
++ /// <reference lib="dom" />
+
+  import {test, expect} from 'bun:test';
+
+  test('dom test', () => {
+    document.body.innerHTML = `<button>My button</button>`;
+    const button = document.querySelector('button');
+    expect(button?.innerText).toEqual('My button');
+  });
+```
+
+Let's run this test with `bun test`:
+
+```bash
+$ bun test
+bun test v0.x.y
+
+dom.test.ts:
+✓ dom test [0.82ms]
+
+ 1 pass
+ 0 fail
+ 1 expect() calls
+Ran 1 tests across 1 files. 1 total [125.00ms]
+```
+
+<!-- ## React Testing Library
+
+Once you've set up `happy-dom` as described above, you can use it with React Testing Library. To get started, install the `@testing-library/react` package as a dev dependency.
+
+```bash
+$ bun add -d @testing-library/react
+``` -->