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/struct_mg_rpc_error.md

@@ -0,0 +1,16 @@
+---
+title: "struct mg_rpc_error"
+decl_name: "struct mg_rpc_error"
+symbol_kind: "struct"
+signature: |
+  struct mg_rpc_error {
+    struct json_token *message;       /* Whole RPC message */
+    struct json_token *id;            /* Message ID */
+    struct json_token *error_code;    /* error.code */
+    struct json_token *error_message; /* error.message */
+    struct json_token *error_data;    /* error.data, can be NULL */
+  };
+---
+
+JSON-RPC error 
+