docs: explain why all setups we set to app

[Arfey]

Add in documentation part (maybe into tutorial) where explaining why all connection, settings and etс, we add to the instance of application.

example:

async def init_pg(app):
    conf = app['config']['postgres']
    engine = await aiopg.sa.create_engine(
        database=conf['database'],
        user=conf['user'],
        password=conf['password'],
        host=conf['host'],
        port=conf['port'],
        minsize=conf['minsize'],
        maxsize=conf['maxsize'],
    )
    app['db'] = engine

Please don't forget:

• add screenshot with your part of documentation
• write below that u want to take it to work

[oktak]

I wonder there exists a website https://demos.aiohttp.org/en/latest/tutorial.html
Is it serving similar purpose? Does this issue aim to merge the two?
Indeed, I am willing to work on this, may be add some screenshots :)
Please let me know more about the direction.

[Arfey]

Tutorial is story about show how may looks project on aiohttp and how u need to design it. The main documentation is about how work and why.

The main goal of this issue it's describe why we choose this way to share settings and etc.

[asvetlov]

Can be a part of #4078 work

[sha016]

@Arfey @asvetlov I'd like to contribute, seems like a good start.

[asvetlov]

Please do