fix: prevent build failure due to cache files

[tomoam]

The following are logs of a build failed in Vercel (these are from folk site for Japanese, but probably the same error occurs in learn.svelte.dev).

23:29:08.034 | Error: ENOENT: no such file or directory, open 'content/tutorial/01-svelte/06-bindings/05-textarea-inputs/README.md'
23:29:08.034 | at Object.openSync (node:fs:600:3)
23:29:08.034 | at Object.readFileSync (node:fs:468:35)
...

This is due to old files still in Vercel.

[vercel]

@tomoam is attempting to deploy a commit to the Svelte Team on Vercel.

A member of the Team first needs to authorize it.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
learn-svelte-dev	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Apr 3, 2023 7:51pm

[Rich-Harris]

thanks!