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_websocket.ezdoc

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+::: cowboy_websocket
+
+The `cowboy_websocket` module implements the Websocket protocol.
+
+The callbacks for websocket handlers are defined in the manual
+for the `cowboy_websocket_handler` behaviour.
+
+:: Types
+
+: close_code() = 1000..4999
+
+Reason for closing the connection.
+
+: frame() = close | ping | pong
+	| {text | binary | close | ping | pong, iodata()}
+	| {close, close_code(), iodata()}
+
+Frames that can be sent to the client.
+
+:: Meta values
+
+: websocket_compress
+
+Type: true | false
+
+Whether a websocket compression extension in in use.
+
+: websocket_version
+
+Type: 7 | 8 | 13
+
+The version of the Websocket protocol being used.
+
+:: Exports
+
+None.