perf: optimize default plugin load speed

[yufeih]

Skip assemblies that don't reference Microsoft.DocAsCode.Plugins. Reduces default plugin assembly load time from ~500ms to 60ms.

#8740

[codecov]

Codecov Report

Patch coverage: 76.92% and project coverage change: +0.01 🎉

Comparison is base (194e503) 76.94% compared to head (586c7b1) 76.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8810      +/-   ##
==========================================
+ Coverage   76.94%   76.95%   +0.01%     
==========================================
  Files         607      607              
  Lines       25058    25069      +11     
==========================================
+ Hits        19280    19291      +11     
  Misses       5778     5778              

Impacted Files	Coverage Δ
...ft.DocAsCode.App/Helpers/DocumentBuilderWrapper.cs	72.97% <76.92%> (+1.13%)	⬆️

... and 2 files with indirect coverage changes

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