Update README to markdown with working badges

[maxrjones]

One of the readme badges was broken, this fixes that while converting to markdown for easier maintenance.

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.33%. Comparing base (eab1742) to head (a473a14).
⚠️ Report is 2 commits behind head on main.

❌ Your project status has failed because the head coverage (99.33%) is below the target coverage (100.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #827   +/-   ##
=======================================
  Coverage   99.33%   99.33%           
=======================================
  Files          26       26           
  Lines         898      898           
=======================================
  Hits          892      892           
  Misses          6        6           

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