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/mbuf.h/struct_mbuf.md

@@ -0,0 +1,14 @@
+---
+title: "struct mbuf"
+decl_name: "struct mbuf"
+symbol_kind: "struct"
+signature: |
+  struct mbuf {
+    char *buf;   /* Buffer pointer */
+    size_t len;  /* Data length. Data is located between offset 0 and len. */
+    size_t size; /* Buffer size allocated by realloc(1). Must be >= len */
+  };
+---
+
+Memory buffer descriptor 
+