Update install instructions for ubuntu, fixing incomplete information

Add some more information about running gmex on ubuntu, namely that you should put the graphics card in performance mode

added 2 commits

• 68541719 - 1 commit from branch main
• 781215c2 - Add some more information about running gmex on ubuntu, namely that you should...

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merged

Hi Joe, FYI: I've canceled the build_geomodel_athena jobs, both pre- and post-merging. We don't need to run them for documentation changes.