Added positronium notes

[andrewfullard]

📝 Description

Type: 📝 documentation

Added some documentation for positronium discussion. Probably won't stay under the "energy input" header, but it's the best place to keep a record.

📌 Resources

See https://en.wikipedia.org/wiki/Positronium
Positronium spectrum https://ui.adsabs.harvard.edu/abs/1949PhRv...75.1696O
SN Ia commentary (section 2.2.2) https://ui.adsabs.harvard.edu/abs/2004ApJ...613.1101M/abstract

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☑️ Checklist

• I requested two reviewers for this pull request
• I updated the documentation according to my changes
• I built the documentation by applying the build_docs label

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[codecov]

Codecov Report

Merging #2107 (033cadb) into master (6269996) will increase coverage by 3.19%.
The diff coverage is n/a.

❗ Current head 033cadb differs from pull request most recent head c90803f. Consider uploading reports for the commit c90803f to get more accurate results

@@            Coverage Diff             @@
##           master    #2107      +/-   ##
==========================================
+ Coverage   58.13%   61.33%   +3.19%     
==========================================
  Files          75       75              
  Lines        8602     8602              
==========================================
+ Hits         5001     5276     +275     
+ Misses       3601     3326     -275     

Impacted Files	Coverage Δ
tardis/visualization/widgets/custom_abundance.py	59.53% <0.00%> (+42.96%)	⬆️

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[tardis-bot]

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[isaacgsmith]

Can we hold off on this until #2105 is merged? That will give us a good location for both the gammaray deposition and the positronium docs.