Update citations document

[drsimonmartin]

Update citations document to explain how to cite the same source in multiple documents when a common bibliography file is used.

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Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (30afdb7) 91.30% compared to head (f1a27a3) 91.30%.
Report is 6 commits behind head on master.

❗ Current head f1a27a3 differs from pull request most recent head a6e9389. Consider uploading reports for the commit a6e9389 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2108   +/-   ##
=======================================
  Coverage   91.30%   91.30%           
=======================================
  Files           7        7           
  Lines         690      690           
=======================================
  Hits          630      630           
  Misses         60       60           

Flag	Coverage Δ
pytests	91.30% <ø> (ø)

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