docs: clarify why we need to use the derived rune

[stephane-vanraes]

In the v4 docs it was clearly explained why we need to use $: to re-evaluate assignments, this is missing for the $derived rune and could be a pitfall for people new to the framework.

Note: should be fixed in the tutorial as well.

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[pkg-pr-new]

pnpm add https://pkg.pr.new/svelte@13919

commit: be931e3

[dummdidumm]

I think it makes sense to mention this, but we shouldn't begin with it.

[stephane-vanraes]

@dummdidumm the current change is basically copypasta from the old docs. Maybe a short explanation can be formatted as note (the ones with the lightbulb) like done in $inspect when mentioning it will be noop'ed in production

[dummdidumm]

thank you!