Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/overview/mbufs.md

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+---
+title: Memory buffers
+---
+
+Each connection has a send and receive buffer, `struct mg_connection::send_mbuf`
+and `struct mg_connection::recv_mbuf` respectively. When data arrives,
+Mongoose appends received data to the `recv_mbuf` and triggers an `MG_EV_RECV`
+event. The user may send data back by calling one of the output functions, like
+`mg_send()` or `mg_printf()`. Output functions append data to the `send_mbuf`.
+When Mongoose successfully writes data to the socket, it discards data from
+`struct mg_connection::send_mbuf` and sends an `MG_EV_SEND` event. When the connection
+is closed, an `MG_EV_CLOSE` event is sent.
+
+![](media/mbuf.png)