networkmanager: Stop using libredirect for building docs

Our gobject-introspection patches make the shared library paths absolute
in the GIR files. When building docs, the library is not yet installed,
though, so we need to replace the absolute path with a local one during build.

Previously we used LD_PRELOAD to load libredirect and rewrite the installed paths
to ones in the build directory. That was unnecessary complicated and many people
spent whole night trying to figure out why it breaks some programs.

Using a symlink from the installed location to the build directory fixes
the issue as well, while having much less moving parts, thus being easier to grasp.
The symlink will be overridden during installation.