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_http_chunk.md

@@ -0,0 +1,11 @@
+---
+title: "mg_printf_http_chunk()"
+decl_name: "mg_printf_http_chunk"
+symbol_kind: "func"
+signature: |
+  void mg_printf_http_chunk(struct mg_connection *nc, const char *fmt, ...);
+---
+
+Send printf-formatted HTTP chunk.
+Functionality is similar to `mg_send_http_chunk()`. 
+