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/.

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+::: Hooks
+
+Cowboy provides two hooks. `onrequest` is called once the request
+line and headers have been received. `onresponse` is called just
+before sending the response.
+
+:: Onrequest
+
+The `onrequest` hook is called as soon as Cowboy finishes fetching
+the request headers. It occurs before any other processing, including
+routing. It can be used to perform any modification needed on the
+request object before continuing with the processing. If a reply is
+sent inside this hook, then Cowboy will move on to the next request,
+skipping any subsequent handling.
+
+This hook is a function that takes a request object as argument,
+and returns a request object. This function MUST NOT crash. Cowboy
+will not send any reply if a crash occurs in this function.
+
+You can specify the `onrequest` hook when creating the listener,
+inside the request options.
+
+``` erlang
+cowboy:start_http(my_http_listener, 100,
+    [{port, 8080}],
+    [
+        {env, [{dispatch, Dispatch}]},
+        {onrequest, fun ?MODULE:debug_hook/1}
+    ]
+).
+```
+
+The following hook function prints the request object everytime a
+request is received. This can be useful for debugging, for example.
+
+``` erlang
+debug_hook(Req) ->
+    erlang:display(Req),
+    Req.
+```
+
+Make sure to always return the last request object obtained.
+
+:: Onresponse
+
+The `onresponse` hook is called right before sending the response
+to the socket. It can be used for the purposes of logging responses,
+or for modifying the response headers or body. The best example is
+providing custom error pages.
+
+Note that like the `onrequest` hook, this function MUST NOT crash.
+Cowboy may or may not send a reply if this function crashes. If a reply
+is sent, the hook MUST explicitly provide all headers that are needed.
+
+You can specify the `onresponse` hook when creating the listener.
+
+``` erlang
+cowboy:start_http(my_http_listener, 100,
+    [{port, 8080}],
+    [
+        {env, [{dispatch, Dispatch}]},
+        {onresponse, fun ?MODULE:custom_404_hook/4}
+    ]
+).
+```
+
+The following hook function will provide a custom body for 404 errors
+when it has not been provided before, and will let Cowboy proceed with
+the default response otherwise.
+
+``` erlang
+custom_404_hook(404, Headers, <<>>, Req) ->
+    Body = <<"404 Not Found.">>,
+    Headers2 = lists:keyreplace(<<"content-length">>, 1, Headers,
+        {<<"content-length">>, integer_to_list(byte_size(Body))}),
+    {ok, Req2} = cowboy_req:reply(404, Headers2, Body, Req),
+    Req2;
+custom_404_hook(_, _, _, Req) ->
+    Req.
+```
+
+Again, make sure to always return the last request object obtained.