Split linting of examples out to a new `pytest-pylint-docstring-examples`

[llucax]

What's needed?

Extract helpers used to test examples and docstrings (sybil, pytest helpers, utilities used in tests) into pytest-pylint-docstring-examples so multiple repositories can reuse them.

Proposed solution

Files to move

• Any small utilities under tests/ or src/ that are used to run or validate examples, e.g. tests/utils.py or specialized fixtures/helpers.

Suggested new repository layout

• frequenz.pytest.examples (package)
  • __init__.py
  • fixtures.py
  • helpers.py

Dependencies

• Typical dev/test deps: pytest, sybil (or other example harness), but these should be listed as extras in the package (e.g., [test]), not required by default.