Rebuild docs to make them easier to maintain

_posts/2000-10-02-callback.md

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+---
+tag: docs
+type: other
+title: Subscription Callback
+---
+
+If the client is used to subscribe to topics, a callback function must be provided in the constructor. This function is called when new messages arrive at the client.
+
+The callback function has the following signature:
+
+    void callback(const char[] topic, byte* payload, unsigned int length)
+
+##### Parameters
+
+ - <span class="methodparams">topic</span> <span class="methodparamstype">const char[]</span> - the topic the message arrived on
+ - <span class="methodparams">payload</span> <span class="methodparamstype">byte[]</span> - the message payload
+ - <span class="methodparams">length</span> <span class="methodparamstype">unsigned int</span> - the length of the message payload
+
+
+Internally, the client uses the same buffer for both inbound and outbound
+messages. After the callback function returns, or if a call to either `publish`
+or subscribe is made from within the callback function, the `topic`
+and `payload` values passed to the function will be overwritten. The application
+should create its own copy of the values if they are required after the callback returns.