Add support for configuring the visibility of the "describedby" link

[bkoelman]

This PR enables configuring whether the "describedby" top-level link is rendered, assuming it has a value.
Follow-up of #1495.

Example (global options):

options.TopLevelLinks = LinkTypes.Self | LinkTypes.Related | LinkTypes.DescribedBy | LinkTypes.Pagination;

Example (overruled per resource type):

[Resource]
[ResourceLinks(TopLevelLinks = LinkTypes.Self | LinkTypes.Related | LinkTypes.DescribedBy | LinkTypes.Pagination)]
public sealed class ExampleResource : Identifiable<long>
{
    // ...
}

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 90.79%. Comparing base (2fcfc65) to head (86ff196).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1516   +/-   ##
=======================================
  Coverage   90.79%   90.79%           
=======================================
  Files         346      346           
  Lines       11062    11063    +1     
  Branches     1813     1814    +1     
=======================================
+ Hits        10044    10045    +1     
  Misses        669      669           
  Partials      349      349           

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