[docs] update the quickstart guide

[iamdejan]

Summary

Make sure that we do not duplicate the installation guide, because:

1. It's redundant; and
2. curl -fsSL https://get.jetify.com/devbox | bash does not work for NixOS-WSL, but using NixOS guide is fine. If other NixOS users come across this page directly and directly install using the installation script, then I'm afraid they cannot install devbox as well.

How was it tested?

N/A