Improve `Installing Direct3D 12 requirements`

**Your Godot version:** 4.6dev5

**Issue description:** GodotEngine switched to d3d for windows builds. The documentation on how to build with directX can be improved: it's offering advanced options without first offering the basic one.

Proposed rework: first and foremost someone should be able to understand how to build with directX. Right now to just install mesa and give the right parameters, you have to skip whole sections about extra advanced stuff, like compiling it yourself, adding extra sdks, and so on.

In my opinion, the information should be organized as follows

1. Use the build-in script (didn't work for me)
2. How to just get a build running (-> download nir libraries, unzip (with clearer instructions about creating a special folder, because the zip comes with 2 inner folders))
3. Advanced/extra options

**URL to the documentation page (if already existing):** https://docs.godotengine.org/en/latest/engine_details/development/compiling/compiling_for_windows.html#doc-compiling-for-windows-installing-d3d12-requirements

Start with the linked “Installing D3D12 requirements” section in compiling_for_windows.html and review the existing build-in script, NIR library, folder, and SDK instructions. Reorganize the page so basic DirectX build setup comes first, followed by the script and advanced options, with clearer zip extraction and folder guidance.

Written by the indexing model from the issue text.