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/net.h/intro.md

@@ -0,0 +1,12 @@
+---
+title: "Core: TCP/UDP/SSL"
+symbol_kind: "intro"
+decl_name: "net.h"
+---
+
+NOTE: Mongoose manager is single threaded. It does not protect
+its data structures by mutexes, therefore all functions that are dealing
+with particular event manager should be called from the same thread,
+with exception of `mg_broadcast()` function. It is fine to have different
+event managers handled by different threads.
+