DOCS: 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.

[welcome]

Thanks for submitting your first pull request! You are awesome! 🤗

If you haven't done so already, check out EBP's Code of Conduct and our Contributing Guide, as this will greatly help the review process.

Welcome to the EBP community! 🎉

[codecov]

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)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[agoose77]

Thanks for this!