Provide installable man pages

make docs: generate Markdown and man pages in doc/
make install-docs: install man pages to be usable directly

Docs are generated from the ezdoc files in doc/src/.

doc/src/manual/cowboy_handler.ezdoc

@@ -0,0 +1,24 @@
+::: cowboy_handler
+
+The `cowboy_handler` middleware executes the handler passed
+through the environment values `handler` and `handler_opts`,
+and adds the result of this execution to the environment as
+the value `result`, indicating that the request has been
+handled and received a response.
+
+Environment input:
+
+* handler = module()
+* handler_opts = any()
+
+Environment output:
+
+* result = ok
+
+:: Types
+
+None.
+
+:: Exports
+
+None.