Remove the confusing doc folder

All documentation is in README.md, guide/ or manual/ now.

doc/README.md

@@ -1,10 +0,0 @@
-Cowboy documentation
-====================
-
-Documentation for Cowboy is available online at the following addresses:
-
- *  [README](http://ninenines.eu/docs/en/cowboy/HEAD/README)
- *  [User Guide](http://ninenines.eu/docs/en/cowboy/HEAD/guide/introduction)
-
-This folder is used for generating the Reference Manual. You can generate
-it yourself by typing `make docs` in the top-level Cowboy directory.

doc/overview.edoc

@@ -1,4 +0,0 @@
-@author Loïc Hoguin <essen@ninenines.eu>
-@copyright 2011-2012 Loïc Hoguin
-@version HEAD
-@title Small, fast, modular HTTP server.