Integrate literalinclude directive for tutorial snippets

[webknjaz]

This issue is a follow-up for #2390 (comment).

Action items:

• Integrate literalinclude to keep example/demos and docs in sync (PR #2396 - Replace copy-pasted code by literalinclude references in docs #2400)
  Here's the result of changes: https://aiohttp.readthedocs.io/en/stable/tutorial.html
• Extend CI to have a separate job for testing specifically those demos and be sure that snippets in docs are working and refer exactly to the source we have in repo, not outdated version of it.

/cc: @asvetlov

P.S. IDK whether I'd have time for this myself either, but at least it's properly documented now.

[jsoucheiron]

First time contributor here. I've added a PR with changes for the tutorials file in the docs. I didn't even try to do the CI part though, hopefully someone with more experience can do that.

Keep the good work, this project is very useful!

[asvetlov]

Thank you @jsoucheiron

[asvetlov]

Fixed by #2400

[webknjaz]

@asvetlov but it does not include demo testing part

[asvetlov]

Oops.

1. Personally I have no spare time to add tests
2. I hope https://github.com/aio-libs/aiohttp-demos will be finished someday and tutorial links will be moved to point on the project

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