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/util.h/mg_avprintf.md

@@ -0,0 +1,22 @@
+---
+title: "mg_avprintf()"
+decl_name: "mg_avprintf"
+symbol_kind: "func"
+signature: |
+  int mg_avprintf(char **buf, size_t size, const char *fmt, va_list ap);
+---
+
+Print message to buffer. If buffer is large enough to hold the message,
+return buffer. If buffer is to small, allocate large enough buffer on heap,
+and return allocated buffer.
+This is a supposed use case:
+
+   char buf[5], *p = buf;
+   p = mg_avprintf(&p, sizeof(buf), "%s", "hi there");
+   use_p_somehow(p);
+   if (p != buf) {
+     free(p);
+   }
+
+The purpose of this is to avoid malloc-ing if generated strings are small. 
+