docs: remove section on callbacks and accessor bindings

[Rich-Harris]

The $effect docs are completely overwhelming. If we want to discourage use of something, we should spend a lot less time talking about it — instead, we currently spend way more time talking about $effect than any other rune.

In particular, I think this section is unnecessary. The example doesn't really resemble anything you'd find in real code, and it makes everything seem a lot more complicated than it needs to be. We should also avoid making up terms like 'writable $derived' without explaining them.

When we update the tutorial to cover Svelte 5, we can afford to spend time in more of these nooks and crannies. Until then, we should remove this.

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[dummdidumm]

Hard disagree. These two specifically were referenced multiple times as a "no don't do it like this, see the docs" in Discord already (the one you want to keep is much more obvious which people don't have as much trouble with). These docs need to be in-depth (for comparison, look at how in-depth the React docs are on useEffect; they have an entire page that is longer than our complete runes page for this). We can certainly reorder things in the final docs, move stuff around (but not into the tutorial), but I vote for keeping it here in the meantime. Not having thorough docs from day 0 means people go down the wrong paths more likely, making them apply workarounds for workarounds etc etc. Putting them on the right track sets them up for success.
Removing it now also means we're much more likely to forget about adding in-depth explanation about this in the final docs.

[benmccann]

$effect has been very overused leading people to talk about "$effect HELL", etc. Since these docs were added I've seen people answer their questions and correct others bad patterns pointing at these docs and think they've done lots of good in tamping down the overuse of $effect. While the tutorial does allow for covering things in a more in depth fashion, I think it'd still fair to include details in the reference docs about when and when not to use a certain rune. And in the short term, before we have such a tutorial, it helps to have our early adopters using better patterns as they answer questions for other users, create content of their own, etc.