Docs: images are in the same dir as index.html

This simplifies writing markdown docs (no need to specify relative path
to images)

PUBLISHED_FROM=039a563b2f7e6a2fbb2a9424ac5315b2496ca184

docs/design-concept/memory-buffers.md

@@ -11,5 +11,5 @@ When Mongoose successfully writes data to the socket, it discards data from
 `struct mg_connection::send_mbuf` and sends `MG_EV_SEND` event. When connection
 is closed, `MG_EV_CLOSE` event is sent.
 
-![](../../static/img/mongoose/mbuf.png)
+![](mbuf.png)