Add Entur specific link on Emission type in transmodel API

[t2gran]

Summary

This PR add Entur specific documentation to the Transmodel API. There is a feature in OTP to enable deployment specific documentation. The documentation added in this PR is only added if the server.apiDocumentationProfile=ENTUR.

Unit tests

This PR only add documentation.

Documentation

This is added to the Transmodel API (only visible at runtime, not in the schema):

Changelog

🟥 Sandbox

Bumping the serialization version id

🟥 Not needed

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.09%. Comparing base (2891fc2) to head (f1cd938).

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #7148      +/-   ##
=============================================
- Coverage      72.09%   72.09%   -0.01%     
+ Complexity     20694    20691       -3     
=============================================
  Files           2244     2244              
  Lines          83899    83899              
  Branches        8347     8347              
=============================================
- Hits           60488    60484       -4     
- Misses         20482    20484       +2     
- Partials        2929     2931       +2     

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