Host legacy playgrounds under /playground/archive/v0.x.x

[berekuk]

• install legacy Squiggle versions in apps/website instead of supporting them in versioned-components
• use each legacy version directly on separate pages
• redirect to that page from /playground when the version is picked in the dropdown, or asked for in ?v=... query parameter

[OAGr]

I wonder if there's some way of doing this with Vercel or something - so we just have different versions of the website at different git points, and never take the old ones down. This would be less convenient for some users, compared to having all of them at one place, but it would also have consistent documentation, and might be easier to maintain.