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>.
Those are commands that act at symbol at point. Documentation often quotes help topics or symbols for which you would like to navigate. 

Documentation commands are available in all doc viewers that I am aware of.  Sometimes you just want a doc and it just happened that you are in the doc viewer buffer. Then what? You will have to close the doc viewer. Navigate to the clj or repl and only then invoke the documentation? This is pretty inconvenient.

Other commands should be there for consistency reasons. If I want to switch to repl, the <kbd>C-c C-z</kbd> is wired so deeply in my brain  that I am clicking them in doc buffer no matter what. This is a good example where cider doesn't have consistent behavior across contexts. There are some others that bother me personally but I will leave those for the following pulls.