Documentation improvements

[mondeja]

• Returns not properly formatted.
• files and args not ordered in pre-commit hooks examples
• Warnings in documentation build (related with sphinx-argparse-cli) (tracked in References must differentiate between upper and lower cases tox-dev/sphinx-argparse-cli#31)
• Some CLI codespans are not properly rendered (related with sphinx-argparse-cli)
• Before using section is confusing, must be rewritten as "The traditional approach". Also, "Translators have to learn Markdown language" should be replaced with "Translators have to learn the whole Markdown language, including details like code blocks..." or something like that
• First disadvantadge in Rationale section is confusing
• Integration with setuptools
• Add contributing section
• CLIs links _<impl>-cli are redundant, links generated by sphinx-argparse-cli should be used instead
• Tips background colors are not consistent with custom theme styles
• Use a tabs extension to show pre-commit hooks examples code blocks like this example.
• Version not shown under logo
• Don't render default values for store_true and store_false CLI actions (tracked in store_true and store_false options shouldn't show default value tox-dev/sphinx-argparse-cli#29)