Move several examples from the docs to tests and then `literalinclude` them

[sobolevn]

We have several quite large examples that live in our docs:

1. https://github.com/dry-python/returns/blob/master/docs/pages/future.rst#futureresult
2. Number example from: https://github.com/dry-python/returns/blob/master/docs/pages/contrib/hypothesis_plugins.rst#check_all_laws
3. https://github.com/dry-python/returns/blob/master/docs/pages/context.rst#requirescontextfutureresult-container

What needs to be done?

• We need to extract these examples from docs to tests/test_docs folder
• Instead, we should add this:

.. literalinclude:: ../../tests/test_docs/test_${SOME_NEW_FILENAME}.py
   :linenos:

• Provide a link to the source code on github somewhere near the example itself
• Possibly we would need to add some dev-dependencies to both pyproject.toml and docs/requirements.txt
• Make sure that it is checked by flake8, mypy, and pytest. And that it actually works.

This way we would have nice, complex, but working examples! 📖

[ssyrota]

Hi, we should paste this :

.. literalinclude:: ../../tests/test_docs/test_${SOME_NEW_FILENAME}.py :linenos:

instead of examples, or only add near. In a nutshell, what result have to be?

[sobolevn]

In a nuthshell, we have to create a similar structure of all of the examples in my original post:

1. Files: https://github.com/dry-python/returns/tree/master/tests/test_examples
2. Includes: https://github.com/dry-python/returns/blob/master/docs/pages/create-your-own-container.rst#step-3-new-interfaces

Do you want to work on this?