Suggest an alternate Qt installer for Windows builds (one with no signup required)

The suggested Qt installation step for Windows requires signing up for their website, and that's something I desperately try to avoid. And I know I'm not alone in that. A possible alternate option is `aqtinstall`, which makes installation even easier since it's just a couple lines in the command prompt.

I think this is what's required to replicate the steps in the windows build guide "Install Qt 5.15" section
```
pip install aqtinstall
aqt install-qt --outputdir c:\Qt windows desktop 5.15.2 win64_msvc2019_64
aqt install-tool --outputdir c:\Qt windows desktop tools_qtcreator
aqt install-tool --outputdir c:\Qt windows desktop tools_qtcreator_kits_and_debuggers
```
This even feels like something that could be added to the cmake configuration, maybe?

Start with the Windows build guide and its "Install Qt 5.15" section. Compare the existing installation steps with the proposed aqtinstall commands, then determine whether the guide or CMake configuration should be updated; done means documenting a no-signup installation path that reproduces the required Qt setup.

Written by the indexing model from the issue text.