docs: add warnings at the top of all example token contracts

[charles-cooper]

discourage people from using them in production (which, apparently people were doing)

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4dd47e3) 89.61% compared to head (6909a6b) 89.90%.
Report is 4 commits behind head on master.

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@@            Coverage Diff             @@
##           master    #3676      +/-   ##
==========================================
+ Coverage   89.61%   89.90%   +0.29%     
==========================================
  Files          80       79       -1     
  Lines       11343    11257      -86     
  Branches     2553     2544       -9     
==========================================
- Hits        10165    10121      -44     
+ Misses        791      747      -44     
- Partials      387      389       +2     

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[fubuloubu]

Could say "not audited"

[charles-cooper]

Could say "not audited"

i think that might imply they are somehow supposed to be production contracts, we just forgot to audit them 😅. they really are sample contracts, and i think people should use other resources if they want templates they can actually use in production.