tutorial: improve table-of-contents accessibility

[jpaquim]

I've mentioned this in the past here: sveltejs/svelte#4673

It's still an issue, using a giant <select> element and requiring JavaScript for basic navigation within the tutorial pages is not a great user experience. I'd expect some kind of actual table-of-contents type of navigation with anchor elements for easier navigation. In particular, it would make it much easier to open different steps in different browser tabs, which would facilitate actually "working with" rather than "working through" the tutorial, and using it more as a reference for certain patterns. I guess after the first run through, the tutorial access pattern should in general switch to random-like, rather than linear.

It's ok if the full ToC is in another (dedicated) page, or within some collapsible section, or similar, tucked away for power users only. It would also help if the breadcrumbs indicating the current section/sub-section were actually usable for navigation rather than only indicative.

[jpaquim]

I think this can be closed with b1cb6c7 being merged, thank you!