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_http_create_digest_auth_header.md

@@ -0,0 +1,13 @@
+---
+title: "mg_http_create_digest_auth_header()"
+decl_name: "mg_http_create_digest_auth_header"
+symbol_kind: "func"
+signature: |
+  int mg_http_create_digest_auth_header(char *buf, size_t buf_len,
+                                        const char *method, const char *uri,
+                                        const char *auth_domain, const char *user,
+                                        const char *passwd);
+---
+
+Create Digest authentication header for client request. 
+