Maintenance cleanup

[blink1073]

• Use Flake8-pyproject to avoid needing a .flake8 file
• Use mdformat instead of prettier for markdown files
• Update to releaser v2
• Use hatch envs for docs, cov, and test
• Add pydata_sphinx_theme
• Add pyproject validation
• Rename readthedocs file for recommended name
• Add rollup test for branch protection
• Other workflow cleanup