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/build-options/enabling-flags.md

@@ -0,0 +1,11 @@
+---
+title: Enabling flags
+---
+
+- `MG_ENABLE_SSL` Enable OpenSSL support
+- `MG_ENABLE_IPV6` Enable IPV6 support
+- `MG_ENABLE_THREADS` enable `mg_start_thread()` API
+- `MG_ENABLE_MQTT_BROKER` enable MQTT broker
+- `MG_ENABLE_DNS_SERVER` enable DNS server
+- `MG_ENABLE_COAP` enable CoAP protocol
+- `MG_ENABLE_GETADDRINFO` enable `getaddrinfo()` in `mg_resolve2()`