contrib to UG: link to hole2 installation instructions

doc/source/contributing_docs.rst

@@ -17,15 +17,6 @@ This guide is about how to contribute to the user guide. If you are looking to a
 Overview
 ========
 
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 Many code examples in the docs are run during the
 doc build. This approach means that code examples will always be up to date,
 but it does make the doc building a bit more complex.
@@ -270,12 +261,7 @@ namely the `hole2`_ program. If you test all the notebooks you may therefore run
 into errors if hole2 is not installed. These errors can be generally ignored unless
 you do specifically want to test the hole2 notebook. Of course, you should take
 note of other errors that occur if hole2 is installed!
-To run the hole2 notebook you'll have to download `hole2`_, compile it, and make sure your system can find
-the hole2 executable. In UNIX-based systems this implies adding its path to the ``$PATH``
-environmental variable like this::
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-    export PATH=$PATH:"<PATH_TO_HOLE2>/exe"
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+To run the hole2 notebook you'll have to install `hole2`_ as described in its documentation. (For Linux, a conda-forge package is available, for other platforms you may have to compile it yourself.)
 
 .. _`hole2`: https://github.com/osmart/hole2