Create cider-doc-map and add "\C" versions of the keys when it makes sense.

Very nice initiative to create a doc map. Thanks. 

This patch adds a couple of control prefixed keys. For my brain <kbd>C-c C-d C-d</kbd> is much more convenient than <kbd>C-c C-d d</kbd>.
No, no. It's an inline require. It's an optional feature. Helm is not loaded by CIDER at startup, but only when the user wants to use cider-cheatsheet and has helm already installed. Otherwise the error is thrown.