[DOC] Add a cookbook entry for an enum validator

[Irallia]

After creating my own enum validator in iGenVar, has @eseiler suggested to add this as a cookbook entry into the seqan3 documentation.
I have collected all the necessary changes for this pr and put them together in a single file.

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Codecov Report

Merging #2695 (2191d71) into master (f5e732d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2695   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
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  Files         276      276           
  Lines       10885    10885           
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  Hits        10693    10693           
  Misses        192      192           

Impacted Files	Coverage Δ
include/seqan3/argument_parser/validators.hpp	91.39% <ø> (ø)

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[smehringer]

@Irallia Can this be moved to sharg? Also (I quickly skimmed the code), coudn't the same be achieved with a value_list_validator?

[Irallia]

We would like to move this to the sharg parser: seqan/sharg-parser#87