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_start_thread.md

@@ -0,0 +1,13 @@
+---
+title: "mg_start_thread()"
+decl_name: "mg_start_thread"
+symbol_kind: "func"
+signature: |
+  void *mg_start_thread(void *(*thread_func);
+---
+
+Start a new detached thread.
+Arguments and semantic is the same as pthead's `pthread_create()`.
+`thread_func` is a thread function, `thread_func_param` is a parameter
+that is passed to the thread function. 
+