[DOC] Add example of custom alphabet for seqan2::Dna

[eseiler]

[seqan-actions]

Documentation preview available at https://docs.seqan.de/preview/seqan/seqan3/3379

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 98.09%. Comparing base (c67fe13) to head (afa6010).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3379   +/-   ##
=======================================
  Coverage   98.09%   98.09%           
=======================================
  Files         270      270           
  Lines       11260    11261    +1     
  Branches      105      105           
=======================================
+ Hits        11046    11047    +1     
  Misses        214      214           

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