Enable setting the "describedby" top-level link

[bkoelman]

This PR enables to plug in a custom IDocumentDescriptionLinkProvider, which is used to set the "describedby" top-level link.

While the default implementation doesn't do anything, this building block is required for our OpenAPI integration.

QUALITY CHECKLIST

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.78%. Comparing base (ae0129e) to head (6db37de).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1495      +/-   ##
==========================================
+ Coverage   90.75%   90.78%   +0.03%     
==========================================
  Files         344      346       +2     
  Lines       11006    11044      +38     
  Branches     1806     1813       +7     
==========================================
+ Hits         9988    10026      +38     
  Misses        669      669              
  Partials      349      349              

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