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>.
I don't want us to make any changes to this keymap yet. I don't think most of the other functions make sense there anyways. I'll have to think more about the direction in which to take this, but I generally prefer to use commands that act on the buffer (like `cider-doc-javadoc`) here instead of commands that act on a symbol (as the notion of symbol is quite unclear here).