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/dns.h/struct_mg_dns_message.md

@@ -0,0 +1,18 @@
+---
+title: "struct mg_dns_message"
+decl_name: "struct mg_dns_message"
+symbol_kind: "struct"
+signature: |
+  struct mg_dns_message {
+    struct mg_str pkt; /* packet body */
+    uint16_t flags;
+    uint16_t transaction_id;
+    int num_questions;
+    int num_answers;
+    struct mg_dns_resource_record questions[MG_MAX_DNS_QUESTIONS];
+    struct mg_dns_resource_record answers[MG_MAX_DNS_ANSWERS];
+  };
+---
+
+DNS message (request and response). 
+