[docs] add DocumenterCitations

[blegat]

Preview https://jump-dev.github.io/SumOfSquares.jl/previews/PR336

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2efff04) 85.10% compared to head (47235c5) 85.10%.

❗ Current head 47235c5 differs from pull request most recent head 1a0cb7a. Consider uploading reports for the commit 1a0cb7a to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #336   +/-   ##
=======================================
  Coverage   85.10%   85.10%           
=======================================
  Files          41       41           
  Lines        2222     2222           
=======================================
  Hits         1891     1891           
  Misses        331      331           

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