Add base documentation of Dispatcher (not ready, not fully documented)

docs/dispatcher/index.md

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+# Overview
+
+Dispatcher is needed for handling incoming updates from Telegram. With dispatcher you can do:
+
+- Handle incoming updates;
+- Filter incoming events before it will be processed by specific handler;
+- Modify event and related data in middlewares;
+- Separate bot functionality between different handlers, modules and packages
+
+Manually dispatcher has the router role and can include another routers.
+
+Dispatcher is also separated into two entities - Router and Dispatcher.
+Dispatcher is subclass of router and should be always is root router.