docs: add link from $host directive docs to custom elements docs

[jamesscottbrown]

This adds a link from the docs for the $host directive to the docs for custom elements. Custom elements are introduced much later, so a link forwards would help someone who is reading the docs sequentially.

[changeset-bot]

⚠️ No Changeset found

Latest commit: abcb3a1

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-15686-svelte.vercel.app/

[github-actions]

Playground

pnpm add https://pkg.pr.new/svelte@15686

[jamesscottbrown]

The link appears on this page of the preview: https://svelte-dev-git-preview-svelte-15686-svelte.vercel.app/docs/svelte/$host

[Rich-Harris]

thank you!