docs: Add dedicated page for custom-setup

Problem/Motivation

It's hard to find custom setup instructions at the moment, they are at https://lupus-decoupled.org/get-started/create-new-project#custom-setup

Proposed resolution

Add a new page "Custom setup" below "Guide" and mentioned it during "Getting started" and maybe "Create a new project" as alternative approach, but below "Get started" I think we want to advertise only "launch online" and "Create new project" with template.