add mountain image; remove redundant encrypt challenge

Author: Kristian Rother

challenges/encrypt.rst

@@ -0,0 +1,34 @@
+Misty Mountains
+---------------
+
+.. image:: mountains.jpg
+
+*Photo by `Rohit Tandon on unsplash.com <https://unsplash.com/@sepoys?utm_content=creditCopyText&utm_medium=referral&utm_source=unsplash>`__*
+  
+**🎯 Find the highest peak.**
+
+You are looking for the dwarven cities.
+They are under the highest mountains, of course.
+As you scan the horizon, you can clearly see the elevation of each mountain:
+
+::
+
+   310
+   464
+   618
+   772
+   926
+   1080
+   ...
+
+A peak is any elevation with two lower numbers before and after it.
+Every peak hosts one of the dwarven cities. You find the elevations in
+:download:`mountains.csv`.
+
+* find the highest peak
+* find all seven peaks
+
+Extra challenge
+---------------
+
+Plot the elevations of the mountains as a line plot.