Port docs to Astro Starlight

[yellowsink]

Vitepress is a lovely docs framework most of the time, but with the addition of shelter UI demos, we've started hitting some realllllly ugly parts of it. Those demos work flawlessly on this branch.

This PR ports the docs over to Astro's Starlight docs framework, which is a very comparable system, but as it is built on Astro makes embedding the solid demos really rather easy.

TODOs:

• Apply our own brand styling to starlight
• Fix plugin styles
• Consider changing the layout or look & feel more drastically than just matching what we did to vp already
• Check over all internal links - the way headers with .s in gets converted to #s is different between vp and sl

Note we lose out on line numbers in the first plugin guide :<

So be it.

[yellowsink]

This also opens the door to move remaining docs code (so, the plugin listing FE) away from Vue.

I'm not bothered about doing that personally, but given how heavily shelter is invested into Solid.js, it is nice to have the option.

[taskylizard]

Switch, SwitchItem, Checkbox, CheckboxItem, Slider (slider track isn't updating), Modal, genId, ErrorBoundary - do not seem to currently work.

use:tooltip on dev also breaks for some reason? openConfirmationModal on dev is immediately cancelled.

[yellowsink]

modals don't work on dev, this is a shui issue not a docs issue, but we should fix it, yes.