Docs: explain MinGW 11.2.0 and tools_mingw90

[ddalcino]

Fix #655

The win64_mingw architecture makes it difficult to know which compiler is required. This change explains how to find out (aqt list-qt windows desktop --long-modules; the answer is "MinGW 11.2.0").

Additionally, the tools_mingw90 tool_name and qt.tools.win64_mingw900 tool_variant are named in a confusing way. This change explains that they contain MinGW 11.2.0, not 9.0.0.

[pzhlkj6612]

Just a note:

File: https://download.qt.io/online/qtsdkrepository/windows_x86/desktop/tools_mingw90/Updates.xml

Content:

<Updates>
    <ApplicationName>{AnyApplication}</ApplicationName>
    <ApplicationVersion>1.0.0</ApplicationVersion>
    <Checksum>true</Checksum>
    <PackageUpdate>
    <Name>qt.tools.win64_mingw900</Name>
    <DisplayName>MinGW 11.2.0 64-bit</DisplayName>
    <Description>MinGW-builds 11.2.0 64-bit toolchain with gcc 11.2.0</Description>
    <Version>9.0.0-1-202203221220</Version>
    <ReleaseDate>2022-03-22</ReleaseDate>
    <Dependencies>qt.tools.qtcreator</Dependencies>
    <Default>false</Default>
    <Script>installscript.qs</Script>
    <SortingPriority>900</SortingPriority>
    <DownloadableArchives>mingw-w64-x86_64-11.2.0-release-posix-seh-rt_v9-rev3.7z</DownloadableArchives>
    <UpdateFile UncompressedSize="599450130" CompressedSize="100757622" OS="Any"/>
    <SHA1>dc95425f544bc8c4e88e997470737b26d2779f98</SHA1>
    </PackageUpdate>
    <SHA1>c7a2ab6650b2f69e422a66e9476b82b6b83f928b</SHA1>
    <MetadataName>2022-03-22-1225_meta.7z</MetadataName>
</Updates>

🙄.