Add docstring checker to pre-commit

[Cjkjvfnby]

Describe your change:

• Add flake8 docstring to the pre-commit hook
• Add global configuration for checks
• Add per-file exclusion to the violation.
• Minor doc fixes

• [ ] Add an algorithm?
• [ ] Fix a bug or typo in an existing algorithm?
• [x] Documentation change?

Checklist:

• [x] I have read CONTRIBUTING.md.
• [ ] This pull request is all my own work -- I have not plagiarized.
• [ ] I know that pull requests will not be merged if they fail the automated tests.
• [ ] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ ] All new Python files are placed inside an existing directory.
• [ ] All filenames are in all lowercase characters with no spaces or dashes.
• [ ] All functions and variable names follow Python naming conventions.
• [ ] All function parameters and return values are annotated with Python type hints.
• [ ] All functions have doctests that pass the automated testing.
• [ ] All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• [ ] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

The script I was using to generate per-file-ignore list. (copy paste pre-commit output into a)

a = '''
ciphers/vigenere_cipher.py:1:1: D100 Missing docstring in public module
ciphers/vigenere_cipher.py:4:1: D103 Missing docstring in public function
'''.strip()

result = {}

for line in a.split("\n"):
    file_name, _, tail = line.partition(":")
    _, _, warning = tail.rpartition(": ")
    code, _, _ = warning.partition(" ")
    result.setdefault(file_name, set()).add(code)

for file_name, errors in sorted(result.items()):
    print(f"    {file_name}: {', '.join(sorted(errors))}")

[cclauss]

Please fix git conflicts.

[Cjkjvfnby]

Every other PR merged could make this one obsolete, because new undocumented things were added.

After merging this one I'll run extra checks to ensure that no unsuppressed warnings left.

[Cjkjvfnby]

Closing it since it became outdated. It would be easy to recreate it from a scratch if needed.