Add a tutorial for cross-compiling a vcpkg port

[AugP]

This is a scenario that is possible with vcpkg but isn't explicitly called out in a tutorial in our docs. Also see: microsoft/vcpkg#39492

[dg0yt]

Note that all the android stuff is cross-building.
(With extra pain on windows hosts, with msys2 in the middle, and the vpckg triplets approach making host tools unaware of target conventions. Most visible in icu.)