Document existing demos

We have 4 demos now but only `polls` is documented.
It would be nice to have something for others too.

I think new docs should not be as comprehensive as polls, no need to describe boring chapters about preparation and folder structure again and again.
Perhaps a page for demo should be enough.

Mentioning all existing demos in docs increase their visibility and teaching effect.

We need volunteers for the task. Sorry, I personally too busy by aiohttp itself.