Draft: Doc update: Build Inkscape for Windows using MSYS2-CI-packages

Building Inkscape from source on Windows is explained in the Windows build instructions in detail. The approach sketched there has, in my opinion, a drawback: Since MSYS2 uses a rolling release model one may end up with a MSYS2 installation containing library versions the Inkscape code will not compile against.

For example, as of today (July 29th 2025), it is impossible to build Inkscape 1.4.x (e.g. to test fixes) in a freshly installed MSYS2 environment with all build dependencies pulled from msys2installdeps.sh (e.g., incompatibilities between libxml2 2.13.8 and GraphicsMagick, see !7351, also most recent poppler libs do not match).

Especially new developers who like to contribute for the first time might be frustrated by failed building attempts.

To overcome this problem I added a section to the Windows build instructions explaining how to pull and use the packaged MSYS2 environments used in the GitLab CI piplines from https://gitlab.com/inkscape/infra/inkscape-ci-windows. In my opinion it produces more reliable results especially when trying to fix or test some stuff in older Inkscape versions. Drawback is that this approach needs more manual interaction by the user compared to the single click version offered so far (and which is still available, of course).

Here is the full documentation: https://gitlab.com/jcwinkler/inkscape/-/blob/a0bff085e18195e51409305502f77e59611b3202/doc/building/windows.md