Getting Started needs updates for Windows

In the **Pixi on Windows** section:

1. There is no mention of [LibPack](https://github.com/FreeCAD/FreeCAD-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).

Start by reading the “Pixi on Windows” section in the Getting Started documentation. Add the LibPack link and the minimal variable setup, then update the Visual Studio guidance after checking whether 2022 or 2026 supports current FreeCAD builds. Done means the section reflects current LibPack and Visual Studio requirements.

Written by the indexing model from the issue text.