Add explanation to all violation messages

[sobolevn]

We need to improve the developer experience. How?

1. There should be a new mode to show error messages turned on by default: detailed one
2. In the detailed mode all violations must contain
   2.1 Link to the docs
   2.2 Solution section from the docs
   2.3 Correct and incorrect examples (we need to lint them in the docs as well)

[sobolevn]

We will need to write a custom formatter:

1. https://flake8.pycqa.org/en/latest/_modules/flake8/formatting/default.html#Default
2. https://flake8.pycqa.org/en/latest/_modules/flake8/main/options.html?highlight=--show-pep8
3. https://github.com/PyCQA/flake8/blob/master/docs/source/plugin-development/formatters.rst

[sobolevn]

https://github.com/astj/flake8-formatter-junit-xml/blob/master/flake8_formatter_junit_xml/formatter.py

[sobolevn]

wemake formatter does that.