Furo: anchor links still broken for page table of contents

[Eric-Arellano]

#340 fixed us not correctly showing the element when going to an anchor link.

But the page table of contents is still wrong with showing what is actively selected:

Whereas if I scroll down the equivalent of our Qiskit top nav bar, it works:

We need to update our visual regression test to ensure this works. We should consider not using a screenshot and instead checking if the expected HTML element has the class for marking it as the currently selected page. That makes the test less sensitive to styling changes.

[Eric-Arellano]

Would be fixed by pradyunsg/furo#664. Fingers crossed that gets accepted 🤞

Otherwise, we will need to figure out how to vendor the JS code, between:

1. Set up webpack in our repo. Which is more complex than it seems. So that we don't break "sdists" (source distributions), we have to make sure that our Python's setup.py knows how to build webpack.
2. Vendor the final webpack output. But it's not very human-understandable.
3. Vendor the raw JavaScript files. They don't have things like modification and rewriting for compatibility, so it makes for a little bit worse performance and less compatibility with old browsers.