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/coap.h/struct_mg_coap_message.md

@@ -0,0 +1,20 @@
+---
+title: "struct mg_coap_message"
+decl_name: "struct mg_coap_message"
+symbol_kind: "struct"
+signature: |
+  struct mg_coap_message {
+    uint32_t flags;
+    uint8_t msg_type;
+    uint8_t code_class;
+    uint8_t code_detail;
+    uint16_t msg_id;
+    struct mg_str token;
+    struct mg_coap_option *options;
+    struct mg_str payload;
+    struct mg_coap_option *optiomg_tail;
+  };
+---
+
+CoAP message. See RFC 7252 for details. 
+