docs: add docs for equation of state

[searchengineorientprogramming]

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[codecov]

Codecov Report

Merging #40 into feature/docs will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           feature/docs      #40   +/-   ##
=============================================
  Coverage         43.45%   43.45%           
=============================================
  Files                 7        7           
  Lines               214      214           
=============================================
  Hits                 93       93           
  Misses              121      121

Impacted Files	Coverage Δ
src/Collections.jl	45.39% <ø> (ø)	⬆️

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