Serve `index.html` when user requests folder path in static file hosting

[jamesmunns]

If we are serving static files and the user GETs a folder path URI like /, we should probably automatically serve /index.html.

[XiNiHa]

Worth mentioning this I guess: https://bjornlu.com/blog/trailing-slash-for-frameworks#servers

[allan2]

I have a setup for this in my Pingora app. PR incoming