Migration guide 2.x -> 3.0 (#1143)

* Initial commit for docs cleanup

* Update migration guide

* More docs

* Added changes description

* Small fixes

docs/dispatcher/errors.rst

@@ -0,0 +1,49 @@
+######
+Errors
+######
+
+
+Handling errors
+===============
+
+Is recommended way that you should use errors inside handlers using try-except block,
+but in common cases you can use global errors handler at router or dispatcher level.
+
+If you specify errors handler for router - it will be used for all handlers inside this router.
+
+If you specify errors handler for dispatcher - it will be used for all handlers inside all routers.
+
+.. code-block:: python
+
+    @router.error(ExceptionTypeFilter(MyCustomException), F.update.message.as_("message"))
+    async def handle_my_custom_exception(event: ErrorEvent, message: Message):
+        # do something with error
+        await message.answer("Oops, something went wrong!")
+
+
+    @router.error()
+    async def error_handler(event: ErrorEvent):
+        logger.critical("Critical error caused by %s", event.exception, exc_info=True)
+        # do something with error
+        ...
+
+
+.. _error-event:
+
+ErrorEvent
+==========
+
+.. automodule:: aiogram.types.error_event
+    :members:
+    :member-order: bysource
+    :undoc-members: True
+    :exclude-members: model_config,model_fields
+
+.. _error-types:
+
+Error types
+===========
+
+.. automodule:: aiogram.exceptions
+    :members:
+    :member-order: bysource