use / as base url instead of host addr in serve cmd

[jcbhmr]

Why? Tunneling! For example GitHub Codespaces will tunnel your port 1234 on your codespace's localhost to https://sometunnel-1234.githubcodespaces.example.org/. if you try to get http://localhost:1234/style.css then it fails since its not on localhost; its on a completely separate domain

ex:

<link rel=stylesheet href="{{ site.base_url }}/style.css">

This pr (hopefully) would fix that by turning the HTML from this:

<link rel=stylesheet href="http://localhost:1234/style.css">

into this which works across localhost and tunnels

<link rel=stylesheet href="/style.css">

[epage]

As I don't deal with tunneling, I overlooked this. As this should still work for local development, I'm fine with this.

[epage]

Thanks!