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/net.h/mg_event_handler_t.md

@@ -0,0 +1,11 @@
+---
+title: "mg_event_handler_t"
+decl_name: "mg_event_handler_t"
+symbol_kind: "typedef"
+signature: |
+  typedef void (*mg_event_handler_t)(struct mg_connection *, int ev, void *);
+---
+
+Callback function (event handler) prototype, must be defined by user.
+Mongoose calls event handler, passing events defined below. 
+