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/c-api/http.h/mg_printf_websocket_frame.md

@@ -0,0 +1,14 @@
+---
+title: "mg_printf_websocket_frame()"
+decl_name: "mg_printf_websocket_frame"
+symbol_kind: "func"
+signature: |
+  void mg_printf_websocket_frame(struct mg_connection *nc, int op_and_flags,
+                                 const char *fmt, ...);
+---
+
+Send websocket frame to the remote end.
+
+Like `mg_send_websocket_frame()`, but allows to create formatted message
+with `printf()`-like semantics. 
+