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

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:

```rst
.. 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! 📖