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
• [x] 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:

• 364

• https://github.com/frequenz-floss/frequenz-sdk-python/pull/384

[llucax]

Examples are now being checked with pylint.