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/json-rpc.h/mg_rpc_create_request.md

@@ -0,0 +1,16 @@
+---
+title: "mg_rpc_create_request()"
+decl_name: "mg_rpc_create_request"
+symbol_kind: "func"
+signature: |
+  int mg_rpc_create_request(char *buf, int len, const char *method,
+                            const char *id, const char *params_fmt, ...);
+---
+
+Create JSON-RPC request in a given buffer.
+
+Return length of the request, which
+can be larger then `len` that indicates an overflow.
+`params_fmt` format string should conform to `json_emit()` API,
+see https://github.com/cesanta/frozen 
+