Remove old docs, rename docs_new to docs

It would be probably good idea to also remove tools/docgen.py and
asciidoc.mk, but asciidoc.mk is still mentioned under `cloud/doc`, which
may contain some useful info which we'll need at least to review before
removing.

PUBLISHED_FROM=faf454d4c52a2f07ea8ac084cf0bd11a0c9c9b3b

docs/design-concept/memory-buffers.md

@@ -0,0 +1,15 @@
+---
+title: Memory buffers
+---
+
+Each connection has 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 `MG_EV_RECV`
+event. 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 `MG_EV_SEND` event. When connection
+is closed, `MG_EV_CLOSE` event is sent.
+
+![](mbuf.png)
+