Upgrade architecture + 5.0 Bot API (#469)
* Moved `methods`, `types` and `client` to root package
* Removed update handler from routers to dispatcher
* Reworked events propagation mechanism to handlers
* Reworked inner middlewares logic (very small change)
* Updated to Bot API 5.0
* Initial migration from MkDocs to Sphinx + config for readthedocs
Resolves#302. We decided to use sentinel pattern (https://python-patterns.guide/python/sentinel-object/) as a solution, but got a few problems with plain `object()`, so instead we use unittest.mock.sentinel and we hope it won't cause side effects.
Most of work done via tg-codegen (https://github.com/aiogram/tg-codegen/pull/1), so it's good to review only implementation of sentinel, processing sentinel in `prepare_parse_mode`, changes in base method model and little test fixes.
