Serve folders by displaying all items below it as links in a page

Right now, it just returns a generic page stating it's a folder.

Good entry point:
https://github.com/krestenlaust/GoWebServer/blob/67e3063ce05c471fd61a069358499c0679aadcb5/request_handers.go#LL81C4-L81C4 (inside `if is folder`)

Start in request_handers.go at the referenced `if is folder` branch and inspect how folder requests currently produce the generic page. Verify the server's folder response and define done as displaying every item below the folder as a link on the page.

Written by the indexing model from the issue text.