(#239): Add cross compiling document for win32

Fixes GNOME/librsvg#239
7 jobs for mingw-cross-build-doc in 52 minutes and 1 second (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #305833
fedora test

00:17:22

 
  Lint
manual #305835
allowed to fail manual
clippy
manual #305834
allowed to fail manual
rustfmt
passed #305836
tarball

00:17:26

 
  Cross Distro
passed #305839
debian_i386:testing

00:13:25

passed #305838
allowed to fail
debian:testing

00:07:53

passed #305837
allowed to fail
fedora:rawhide

00:17:12