Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/overview/intro.md

@@ -0,0 +1,40 @@
+---
+title: Overview
+items:
+#  - { name: usage-example.md }
+  - { name: concept.md }
+  - { name: mbufs.md }
+  - { name: event-handler.md }
+  - { name: events.md }
+  - { name: conn-flags.md }
+  - { name: build-options }
+  - { name: usage-example.md }
+---
+
+Mongoose is a networking library written in C.
+It is a swiss army knife for embedded network programming.
+It implements event-driven non-blocking APIs for TCP, UDP, HTTP,
+WebSocket, CoAP, MQTT for client and server mode.
+Features include:
+
+- Cross-platform: works on Linux/UNIX, MacOS, QNX, eCos, Windows, Android,
+  iPhone, FreeRTOS
+- Native support for [PicoTCP embedded TCP/IP stack](http://www.picotcp.com),
+  [LWIP embedded TCP/IP stack](https://en.wikipedia.org/wiki/LwIP)
+- Works on a variety of embedded boards: TI CC3200, TI MSP430, STM32, ESP8266;
+  on all Linux-based boards like Raspberry PI, BeagleBone, etc
+- Single-threaded, asynchronous, non-blocking core with simple event-based API
+- Built-in protocols:
+   - plain TCP, plain UDP, SSL/TLS (one-way or two-way), client and server
+   - HTTP client and server
+   - WebSocket client and server
+   - MQTT client and server
+   - CoAP client and server
+   - DNS client and server
+   - asynchronous DNS resolver
+- Tiny static and run-time footprint
+- Source code is both ISO C and ISO C++ compliant
+- Very easy to integrate: just copy
+  [mongoose.c](https://raw.githubusercontent.com/cesanta/mongoose/master/mongoose.c) and
+  [mongoose.h](https://raw.githubusercontent.com/cesanta/mongoose/master/mongoose.h)
+  files to your build tree