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 :white_check_mark:

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

:exclamation: 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

```diff @@ Coverage Diff @@ ## master #2108 +/- ## ======================================= Coverage 91.30% 91.30% ======================================= Files 7 7 Lines 690 690 ======================================= Hits 630 630 Misses 60 60 ``` | [Flag](https://app.codecov.io/gh/executablebooks/jupyter-book/pull/2108/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | Coverage Δ | | |---|---|---| | [pytests](https://app.codecov.io/gh/executablebooks/jupyter-book/pull/2108/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks) | `91.30% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=executablebooks#carryforward-flags-in-the-pull-request-comment) to find out more.

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