Docs: Add an intro for how to use `harper.js` inside a Chrome extension

I'm not a browser extension expert but I've stumbled through making a couple by following howtos or guidance of an AI.

But getting `harper.js` to work inside one seems to be next level complicated. I'm only using Google Search AI because I don't have tokens for a coding AI but every time it says we're done there's something wrong that's outside my areas of expertise.

Maybe it's obvious to a webdev or someone who's made lots of Chrome Extensions.

I was going to make a little tool similar to [harper-glasses](https://github.com/hippietrail/harper-glasses) but instead of linting the selected text, called the tokenization API and annotates the tokens in the selection with POS tags.

But a tiny example either as part of the Harper docs that just opened a popup/modal and displays the harper-core version it's built with would be nice and minimal perhaps.

I also find the [harper.js API Reference](https://writewithharper.com/docs/harperjs/ref/index.html) very opaque. It seems to list only objects and not functions. For instance how to I find the function to get the `harper-core` version? I'm sure it exists.

Start with the Harper documentation and the harper.js API Reference, then compare the proposed use case with the harper-glasses example. Document the steps for using harper.js in a Chrome extension and add a minimal popup or modal example that displays the harper-core version; clarify the tokenization API and function lookup where the reference is opaque.

Written by the indexing model from the issue text.