Updated navigation to include all_exercises_challenges.
Updated navigation to include all_exercises_challenges.

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