This creates `GioUnix`, `GioWin32`, `GLibUnix` and `GLibWin32`. These
bodies of documentation are in addition to the main, platform agnostic,
documentation for both libraries.

This commit necessarily includes various mechanical changes to update
the repository namespace used in various existing documentation links to
platform specific APIs.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037