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>.
Yes, I like that. `cider-docview-mode`, `cider-docview-map`, `cider-docview-emphasis-face`, etc. Sounds good, indeed!