docs: move $host docs to custom elements page

[benmccann]

I think this makes the docs more approachable. If you're learning about custom elements it's more helpful to have everything on a single page vs split across two pages. And if you're learning about Svelte in general and never using custom elements then you don't need to see $host in the sidebar, which makes it feel like there are more concepts to learn

The one case where this is slightly worse is if you see $host in your codebase and want to know what it does. But you can find it very quickly via search and I'm not sure we really need everything in the sidebar (e.g. we don't have $effect.pre in the sidebar, which is probably used a lot more)

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15562-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15562

[Rich-Harris]

I think it's better the way it is now. It's reference documentation; it would be weird if the 'runes' section didn't contain all the runes

[benmccann]

Yeah, fair enough. I might take a stab at separating it into more of a guide vs reference like we have in the SvelteKit docs

[Rich-Harris]

Anecdotally I've heard people say that the SvelteKit docs are trash compared to the Svelte docs, so we would want to be extremely cautious before embarking on something like that