[v2] docs

We need to update docs to show how to use Prism v2.

@JaKXz @RunDevelopment What does an example Prism v2 usage look like?

So far this didn't work for me:

<link rel="stylesheet" href="./node_modules/prismjs/dist/themes/prism.css" />
<script type="module" src="./main.js"></script>

<pre><code class="language-js">
import { Prism } from "./node_modules/prismjs/dist/esm/core.js"

const prism = new Prism()

prism.highlightAll()
</code></pre>

// main.js
import { Prism } from "./node_modules/prismjs/dist/esm/core.js"

const prism = new Prism()

prism.highlightAll()

Repo: https://github.com/trusktr/prism-v2-test

Use the HTML example and main.js from the issue as the starting point, then compare them with the linked prism-v2-test repository. Confirm the supported Prism v2 usage and update the relevant documentation; done when a working example is documented.