Add documentation of the concepts behind PLONK

[moCello]

[codecov[bot]]

Codecov Report

Merging #628 (a463fae) into master (282221d) will decrease coverage by 0.00%. The diff coverage is n/a.

:exclamation: Current head a463fae differs from pull request most recent head 7c75cb9. Consider uploading reports for the commit 7c75cb9 to get more accurate results

@@            Coverage Diff             @@
##           master     #628      +/-   ##
==========================================
- Coverage   97.71%   97.71%   -0.01%     
==========================================
  Files          54       54              
  Lines        4067     4066       -1     
==========================================
- Hits         3974     3973       -1     
  Misses         93       93              

Impacted Files	Coverage Δ
src/lib.rs	100.00% <ø> (ø)
src/permutation.rs	99.30% <0.00%> (-0.01%)	:arrow_down:
src/fft/evaluations.rs	100.00% <0.00%> (ø)
src/proof_system/widget.rs	98.58% <0.00%> (ø)
src/constraint_system/composer.rs	98.05% <0.00%> (ø)
src/fft/polynomial.rs	96.20% <0.00%> (+0.04%)	:arrow_up:

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