Add index page for multiple slides build

[jaanli]

Describe the bug

Cannot host multiple entries.

Minimal reproduction

Steps to reproduce the behavior:

1. Go to documentation: https://sli.dev/guide/hosting#multiple-builds
2. Edit your build command in Vercel

3. Verify that the build works locally
4. Verify that the build fails and that no index is created.

To reproduce with code, see here:

https://github.com/onefact/talks.onefact.org (and see the dist directory from vite to see an example of how there is no index page from which to access multiple slide decks!)

Environment

• Slidev version: latest (https://github.com/onefact/talks.onefact.org has the package-lock.json file if needed for specifics)
• Browser: chrome / safari / firefox
• OS: Sonoma 14.5 MBP

[KermanX]

In fact, currently, Slidev doesn't create an index page for your slides. slidev build a.md b.md is the same as slidev build a.md --out dist/a/ && slidev build b.md --out dist/b/. In your repo, it seems that the dist directory wasn't cleared after previous builds.

About how to host multiple slides, this discussion may help: #1777 (comment)