docs: fix secret file reference

[phbelitz]

With the switch to Golang the authentication secrets require to be named secret.yaml instead of the previous auth.yaml. This has been fixed in the documentation.

Checklist

• PR is rebased to/aimed at branch develop
• PR follows Contributing Guide
• Added tests (if necessary)
• Extended README/Documentation (if necessary)
• Adjusted versions of image and Helm chart in Chart.yaml (if necessary)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.88%. Comparing base (1562cd9) to head (e309975).
Report is 9 commits behind head on develop.

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1625      +/-   ##
===========================================
- Coverage    88.08%   87.88%   -0.20%     
===========================================
  Files           36       35       -1     
  Lines         2509     2468      -41     
===========================================
- Hits          2210     2169      -41     
  Misses         255      255              
  Partials        44       44              

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