Added specific links to Linux, Mac, Windows in drop-down nav.
Added specific links to Linux, Mac, Windows in drop-down nav.

BadWizard/intro_programming,ehmatthes/intro_programming,chenleo/ipynotebook,ehmatthes/intro_programming,chenleo/ipynotebook,chenleo/ipynotebook,BadWizard/intro_programming,nntisapeh/intro_programming,nntisapeh/intro_programming,BadWizard/intro_programming,BadWizard/intro_programming,ehmatthes/intro_programming,nntisapeh/intro_programming,chenleo/ipynotebook,ehmatthes/intro_programming,nntisapeh/intro_programming