Move mongoose docs to https://cesanta.com/docs/

PUBLISHED_FROM=ebf5568abe82952ab2751298185a10189098013f

docs/overview/event-handler.md

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-title: Event handler function
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+# Event handler function
 
 Each connection has an event handler function associated with it. That function
 must be implemented by the user. Event handler is the key element of the Mongoose