Migrate some docs to Markdown

[prkumar]

• Remove AUTHORS.md in favor of using pyproject.toml as single source of truth
• Update CONTRIBUTING.md to direct new contributors to add themselves to pyproject.toml
• Convert documentation files from RST to MD:
  • CHANGELOG.rst → CHANGELOG.md
  • CONTRIBUTING.rst → CONTRIBUTING.md
  • README.rst → README.md

[leiserfg]

LGTM! The docs themselves need some updates too, but that can be in another PR.