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/http.h/mg_get_http_var.md

@@ -0,0 +1,16 @@
+---
+title: "mg_get_http_var()"
+decl_name: "mg_get_http_var"
+symbol_kind: "func"
+signature: |
+  int mg_get_http_var(const struct mg_str *buf, const char *name, char *dst,
+                      size_t dst_len);
+---
+
+Fetch an HTTP form variable.
+
+Fetch a variable `name` from a `buf` into a buffer specified by
+`dst`, `dst_len`. Destination is always zero-terminated. Return length
+of a fetched variable. If not found, 0 is returned. `buf` must be
+valid url-encoded buffer. If destination is too small, `-1` is returned. 
+