Fix models/code_scanning.py: add 'documentation' to Classification enum

[gb-friotte]

What

Added 'documentation' to Classification Enum.

Why

Using code_scanning.organization_alerts resulted in a ModelError.

References

https://jira.greenbone.net/browse/DEVOPS-856

Checklist

• code_scanning.organization_alerts ran successfully after calling it locally after the fix

[github-actions]

Conventional Commits Report

Type	Number
Bug Fixes	1

🚀 Conventional commits found.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (78d852d) 90.38% compared to head (e6626d9) 90.38%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #926   +/-   ##
=======================================
  Coverage   90.38%   90.38%           
=======================================
  Files          97       97           
  Lines        6782     6783    +1     
  Branches      976      976           
=======================================
+ Hits         6130     6131    +1     
  Misses        446      446           
  Partials      206      206           

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