Improve DKG engine package documentation

[jordanschalm]

• Place documentation comments directly above package declaration, so it is interpreted by godoc as package documentation (not just a comment)
• Align diagram components regardless of tab size
• Add links and headers
• Minor improvements to text

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.23%. Comparing base (b3d0864) to head (12d1320).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6758      +/-   ##
==========================================
- Coverage   41.24%   41.23%   -0.02%     
==========================================
  Files        2061     2061              
  Lines      182687   182687              
==========================================
- Hits        75358    75338      -20     
- Misses     101019   101038      +19     
- Partials     6310     6311       +1     

Flag	Coverage Δ
unittests	41.23% <ø> (-0.02%)	⬇️

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