Simplify doc generation infra

PUBLISHED_FROM=80e77fda1c358294f251ca77d3dd64e8ca9160e5

docs/intro.md

@@ -0,0 +1,14 @@
+---
+title: Mongoose - a networking library
+color: '#1D6482'
+repo: https://github.com/cesanta/mongoose
+items:
+  - { type: dir, name: usage-example }
+  - { type: dir, name: design-concept }
+  - { type: dir, name: build-options }
+  - { type: flat, name: c-api }
+---
+
+Mongoose is a multi-protocol embedded networking library with
+that implements TCP, UDP, HTTP, WebSocket, CoAP, MQTT, JSON-RPC,
+both client and server mode.