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_std_error.md

@@ -0,0 +1,22 @@
+---
+title: "mg_rpc_create_std_error()"
+decl_name: "mg_rpc_create_std_error"
+symbol_kind: "func"
+signature: |
+  int mg_rpc_create_std_error(char *buf, int len, struct mg_rpc_request *req,
+                              int code);
+---
+
+Create JSON-RPC error in a given buffer.
+
+Return length of the error, which
+can be larger then `len` that indicates an overflow. See
+JSON_RPC_*_ERROR definitions for standard error values:
+
+- `#define JSON_RPC_PARSE_ERROR (-32700)`
+- `#define JSON_RPC_INVALID_REQUEST_ERROR (-32600)`
+- `#define JSON_RPC_METHOD_NOT_FOUND_ERROR (-32601)`
+- `#define JSON_RPC_INVALID_PARAMS_ERROR (-32602)`
+- `#define JSON_RPC_INTERNAL_ERROR (-32603)`
+- `#define JSON_RPC_SERVER_ERROR (-32000)` 
+