Fail docfx on invalid internal link

[bkoelman]

References:

• [Feature Request] Turn off warning about project reference dotnet/docfx#9804
• Option to treat 'Invalid file link' as error instead of warning dotnet/docfx#7433
• --warningsAsErrors does not work dotnet/docfx#5224

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• N/A: Adapted tests
• Documentation updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.69%. Comparing base (1e8275c) to head (5e9b78c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1681   +/-   ##
=======================================
  Coverage   90.69%   90.69%           
=======================================
  Files         354      354           
  Lines       11536    11536           
  Branches     1900     1900           
=======================================
  Hits        10462    10462           
  Misses        702      702           
  Partials      372      372           

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