Document existing demos

[asvetlov]

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.

[gyermolenko]

Regarding docs themselves: definitely good to have. Just to mention: polls is intended as aiohttp tutorial. Others can choose much simpler style.

p.s. There is work to be done to make all demos consistent (async factories returning app only; at least a smoke test for each, for deprecation or syntax errors; same configuration principles maybe).

[asvetlov]

I suspect until new demos are mentioned in https://demos.aiohttp.org/en/latest/ somehow they don't exist for the most part of aiohttp users. People just cannot understand where and what to search. @jettify @gyermolenko could you put your energy on it?

I think a simple catalog of existing examples is enough. Every project has own README, we can reuse it.

[Arfey]

As example, we can do it like this https://reactjs.org/community/examples.html List of name - short description. (without structure of project, only link to GitHub)

[asvetlov]

Looks good