PEP257 docstrings for file "./docs/conf.py"

[insspb]

Cover ./docs/conf.py file with docstrings and follow PEP257. We use pydocstyle for validation.

Current validation log:

./docs/conf.py:1 at module level:
        D100: Missing docstring in public module
./docs/conf.py:28 in public class `Mock`:
        D101: Missing docstring in public class
./docs/conf.py:29 in public method `__init__`:
        D107: Missing docstring in __init__
./docs/conf.py:32 in public method `__call__`:
        D102: Missing docstring in public method
./docs/conf.py:36 in public method `__getattr__`:
        D105: Missing docstring in magic method

Subtask for #742