Getting Started needs updates for Windows

[pjcreath]

In the Pixi on Windows section:

1. There is no mention of LibPack. That should be front and center. It's probably also worth incorporating the minimal set of steps need to use LibPack (such as setting the variables).

2. The instructions describe installing Visual Studio Community 2019. That only supports LibPack up to v2.11, which was for FreeCAD 0.21. It does not work with current FreeCAD builds. The instructions should point to either 2022 (which is known to work) or 2026 (if it works -- just released).