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/mbuf_insert.md

@@ -0,0 +1,14 @@
+---
+title: "mbuf_insert()"
+decl_name: "mbuf_insert"
+symbol_kind: "func"
+signature: |
+  size_t mbuf_insert(struct mbuf *, size_t, const void *, size_t);
+---
+
+Insert data at a specified offset in the Mbuf.
+
+Existing data will be shifted forwards and the buffer will
+be grown if necessary.
+Return the number of bytes inserted. 
+