docs: fix preview docs deployments

[teemingc]

We need to update the explicit environment variables docs to solve the preview error deployment. It's looking for $app/environment types but they've been removed when we merged main in https://github.com/sveltejs/svelte.dev/actions/runs/27088756592/job/79948243995#step:10:25

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[teemingc]

There's probably a better way to introduce $app/env explicit environment variables

[vercel]

Additional Suggestion:

Documentation links to deleted $app-environment reference file, causing a broken link.

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15975-svelte.vercel.app/

[vercel]

Additional Suggestions:

1. Documentation contains broken links to deleted $env-static-private reference file after SvelteKit 3 migration

2. Documentation contains broken links to deleted $env-static-private reference file

[teemingc]

Requires merging sveltejs/svelte.dev#2031 to get the deployment working

[Rich-Harris]

Gah, still failing — needs $app/env/private to be defined in the adapter-vercel docs, and the February 2024 'what's new' blog post links to a missing /docs/kit/$env-static-public...

[teemingc]

Alright I've added the ambient type and restored the deleted pages. We can probably remove them when svelte.dev uses the version-3 branch as the default and add redirects to the server handle hook there