Implemented a MediaGroupBuilder class in 'aiogram/utils/media_group.py' to help construct media groups. The class supports addition of different media types (audio, photo, video, document) to the media group with a maximum limit of 10 files. The functionality is demonstrated and usage is documented in 'docs/utils/media_group.rst'. Added related test cases in 'tests/test_utils/test_media_group.py'. This is to streamline and simplify the process of media group creation
