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

docs/dispatcher/observer.md

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+# Observer
+
+Observer is used for filtering and handling different events. That is part of internal API with some public methods and is recommended to don't use methods is not listed here.
+
+In `aiogram` framework is available two variants of observer:
+
+1. [EventObserver](#eventobserver) 
+1. [TelegramEventObserver](#telegrameventobserver)
+
+
+## EventObserver
+Reference: `#!python3 aiogram.dispatcher.event.observer.EventObserver`
+
+That is base observer for all events.
+
+### Base registering method
+Method: `<observer>.register(callback, filter1, filter2, ...)`
+
+| Argument | Type | Description |
+| --- | --- | --- |
+| `callback` | `#!python3 Callable[[Any], Awaitable[Any]]` | Event handler |
+| `*filters` | `#!python3 Union[Callable[[Any], Any], Callable[[Any], Awaitable[Any]], BaseFilter]` | Ordered filters set |
+
+Will return original callback.
+
+
+### Decorator-style registering method
+
+Usage:
+```python3
+@<observer>(filter1, filter2, ...)
+async def handler(*args, **kwargs):
+    pass
+```
+
+## TelegramEventObserver
+Is subclass of [EventObserver](#eventobserver) with some differences.
+In this handler can be bounded filters which can be used as keyword arguments instead of writing full references when you register new handlers.
+
+### Registering bound filters
+
+Bound filter should be subclass of [BaseFilter](filters/base_filter.md)
+
+`#!python3 <observer>.bind_filter(MyFilter)`
+
+### Registering handlers
+Method: `EventObserver.register(callback, filter1, filter2, ..., bound_filter=value, ...)`
+In this method is added bound filters keywords interface.
+
+| Argument | Type | Description |
+| --- | --- | --- |
+| `callback` | `#!python3 Callable[[Any], Awaitable[Any]]` | Event handler |
+| `*filters` | `#!python3 Union[Callable[[Any], Any], Callable[[Any], Awaitable[Any]], BaseFilter]` | Ordered filters set |
+| `**bound_filters` | `#!python3 Any` | Bound filters |