cleanUrls does not check for nested index.md

[MarkusKeck]

Describe the bug

Vitepress should check for a nested index.md if no file for the current url exists

Reproduction

1. Create a folder test
2. Create a index.md file inside the test folder

Call the URL http://localhost:5173/test

http://localhost:5173/test -> 404 page not found
http://localhost:5173/test/ -> index.md gets found

This behavior makes sense from a file and folder perspective, but not from an url perspective.
URLs should point to the same resource regardless of using a trailing slash or not

I thought this gets handled with the cleanUrls setting
https://vitepress.vuejs.org/config/app-configs#cleanurls-experimental

In my opinion with-subfolders should check for an index.md file and without-subfolders should not.

Expected behavior

If no file gets found for the currently called url vitepress should check if the called folder contains an index file

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (32) x64 AMD Ryzen 9 3950X 16-Core Processor
    Memory: 44.27 GB / 63.91 GB
  Binaries:
    Node: 16.14.2 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.19 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 8.7.0 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Edge: Spartan (44.19041.1266.0), Chromium (108.0.1462.76)
    Internet Explorer: 11.0.19041.1566
  npmPackages:
    vitepress: ^1.0.0-alpha.13 => 1.0.0-alpha.35

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.

[brc-dd]

We actually attempted to fix this in #1575. But looks like I missed the opposite case, I'll create a PR soon. However, it is worth noting that the fix doesn't work in dev, it works only in prod, and whether to redirect from trailing slash to non-trailing slash, the choice is up to you, you'll need to configure your hosting provider accordingly. Also, page relative URLs (incl. assets if any) might break as it is a browser feature (will be resolved differently depending if one has trailing slash or not).