Lint all examples in docstrings

[leandro-lucarella-frequenz]

We should run all the same linting tools that we use for the source code on the examples in docstrings:

• black
• isort
• pylint
• mypy
• pydocstyle
• darglint

We should probably support a way to pass extra configuration when applying to examples, as they are not full pieces of code.

[leandro-lucarella-frequenz]

First step towards this:

• Validate docstring examples #364
• Validate docstring examples #384

[llucax]

Examples are now being checked with pylint.