docs: clean up and add a real data XRD example

[john-halloran]

Closes #128.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.50%. Comparing base (d7586bc) to head (18cf53c).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #191   +/-   ##
=======================================
  Coverage   77.50%   77.50%           
=======================================
  Files           3        3           
  Lines          40       40           
=======================================
  Hits           31       31           
  Misses          9        9           

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