Adds docs for various similar level columns

[andrewfullard]

Also moved notation to a more accessible spot.

📝 Description

Type: 📝 documentation

Attempted to write some docs about the differences between level index, ID and number. Also moved notation to a better location. Fixes issue #258 and #364

🚦 Testing

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☑️ Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.30%. Comparing base (09bd62c) to head (d3c29fb).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #407   +/-   ##
=======================================
  Coverage   58.30%   58.30%           
=======================================
  Files          30       30           
  Lines        2758     2758           
=======================================
  Hits         1608     1608           
  Misses       1150     1150           

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