Update links to broken destinations in `learn.svelte.dev/vercel.json`

[benmccann]

https://github.com/sveltejs/svelte.dev/blob/main/apps/learn.svelte.dev/vercel.json contains a number of URLs that do not exist. Hopefully we can point most of these at something semi-relevant such as https://svelte.dev/docs/svelte/legacy-slots for the slots one or whatever is most appropriate for each

https://svelte.dev/tutorial/svelte/event-forwarding
https://svelte.dev/tutorial/svelte/marked
https://svelte.dev/tutorial/svelte/onmount
https://svelte.dev/tutorial/svelte/update
https://svelte.dev/tutorial/svelte/elizabot
https://svelte.dev/tutorial/svelte/tick
https://svelte.dev/tutorial/svelte/writable-stores
https://svelte.dev/tutorial/svelte/auto-subscriptions
https://svelte.dev/tutorial/svelte/readable-stores
https://svelte.dev/tutorial/svelte/derived-stores
https://svelte.dev/tutorial/svelte/custom-stores
https://svelte.dev/tutorial/svelte/store-bindings
https://svelte.dev/tutorial/svelte/animate
https://svelte.dev/tutorial/svelte/core
https://svelte.dev/tutorial/svelte/tippy.js
https://svelte.dev/tutorial/svelte/slots
https://svelte.dev/tutorial/svelte/named-slots
https://svelte.dev/tutorial/svelte/slot-fallbacks
https://svelte.dev/tutorial/svelte/slot-props
https://svelte.dev/tutorial/svelte/optional-slots
https://svelte.dev/tutorial/svelte/svelte-self
https://svelte.dev/tutorial/svelte/svelte-component
https://svelte.dev/tutorial/svelte/svelte-options
https://svelte.dev/tutorial/svelte/svelte-fragment
https://svelte.dev/tutorial/svelte/debug
https://svelte.dev/tutorial/kit/xx-custom-error-messages