OpenAPI: Set the describedby top-level link

[bkoelman]

Closes #1456.

After adding the next lines to JsonApiDotNetCoreExample\Program.cs:

builder.Services.Configure<SwaggerOptions>(
    options => options.RouteTemplate = "api-docs/{documentName}/openapi.yaml");

// ...

app.UseSwaggerUI(options => options.SwaggerEndpoint("/api-docs/v1/openapi.yaml", "ExampleAPI V1"));

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• Documentation updated

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 90.79%. Comparing base (22b00b3) to head (6b5bb6a).

Files	Patch %	Lines
...tNetCore.OpenApi/OpenApiDescriptionLinkProvider.cs	84.61%	1 Missing and 1 partial ⚠️
...re.OpenApi/JsonApiRequestFormatMetadataProvider.cs	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           openapi    #1497      +/-   ##
===========================================
- Coverage    90.79%   90.79%   -0.01%     
===========================================
  Files          393      394       +1     
  Lines        12938    12957      +19     
  Branches      2055     2057       +2     
===========================================
+ Hits         11747    11764      +17     
- Misses         776      777       +1     
- Partials       415      416       +1     

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