doc: cross-doc absolute link hack

This is an ugly hack to also resolve the link when the document is
rendered in GitHub, where it is likely to be browsed, because it is
the community standard location for a GitHub repo's contributing
docs.

Coordinate with #1849 to better separate RTD docs with GitHub docs
in the future.

Signed-off-by: Lukas Puehringer <[email protected]>

Author: lukpueh

docs/CONTRIBUTING.rst

@@ -11,7 +11,7 @@ and must be `unit tested <#unit-tests>`_.
 
 .. note::
 
-     Also see :ref:`development installation instructions <installation:Install for development>`.
+     Also see `development installation instructions <https://theupdateframework.readthedocs.io/en/latest/INSTALLATION.html#install-for-development>`_.
 
 Testing
 =======