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_vprintf.md

@@ -0,0 +1,10 @@
+---
+title: "mg_vprintf()"
+decl_name: "mg_vprintf"
+symbol_kind: "func"
+signature: |
+  int mg_vprintf(struct mg_connection *, const char *fmt, va_list ap);
+---
+
+Same as `mg_printf()`, but takes `va_list ap` as an argument. 
+