Refer to the "Getting started" guide on hugo-kde's wiki

I'm in the process of writing more docs for our Hugo theme. Recently I rewrote the "Getting started" page and also added some details from this repo's README. The aim is to make it suitable as a "getting started" guide for all website projects that use the theme.

This MR proposes removing some parts of the README and just referring to that guide at https://invent.kde.org/websites/hugo-kde/-/wikis/Getting-started. Please have a look.