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_next.md

@@ -0,0 +1,20 @@
+---
+title: "mg_next()"
+decl_name: "mg_next"
+symbol_kind: "func"
+signature: |
+  struct mg_connection *mg_next(struct mg_mgr *, struct mg_connection *);
+---
+
+Iterate over all active connections.
+
+Returns next connection from the list
+of active connections, or `NULL` if there is no more connections. Below
+is the iteration idiom:
+
+```c
+for (c = mg_next(srv, NULL); c != NULL; c = mg_next(srv, c)) {
+  // Do something with connection `c`
+}
+``` 
+