📚 [index] Convert to Markdown using MyST

[cjolowicz]

• 📚 [codeofconduct] Convert to Markdown using MyST
• 📚 [contributing] Use YAML front matter in include directive
• 📚 [index] Convert to Markdown using MyST
• 📚 [css] Remove unused stylesheet and unsupported rst-class directive
• 📚 [index] Use YAML front matter in toctree directive
• 📚 [README] Avoid name conflict in black and pre-commit references
• 📚 [index] Use native include directive for badges and features