Add missing docs and enable flake8 corresponding checks

[lucekdudek]

• Enable Flake8's extend in pyproject.toml
• Make checks_codestyle pass by adding missing docs

Extends:

• D100 - Add missing docs to public modules
• D101 - Add missing docs to public classes
• D102- Add missing docs to public methods
• D104 - Add missing docs to public packages
• D105 - Add missing docs to magic methods
• D107 - Add missing docs to init
• D400 - End first lines of docs with periods
• D403 - Capitalize first words of docs