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_base64_encode.md

@@ -0,0 +1,12 @@
+---
+title: "mg_base64_encode()"
+decl_name: "mg_base64_encode"
+symbol_kind: "func"
+signature: |
+  void mg_base64_encode(const unsigned char *src, int src_len, char *dst);
+---
+
+Base64-encode chunk of memory `src`, `src_len` into the destination `dst`.
+Destination has to have enough space to hold encoded buffer.
+Destination is '\0'-terminated. 
+