feat: add output config overridable by --output

[yufeih]

• add a new output JSON config to build and pdf command overridable by --output command line args
• make docfx init generate output as _site and dest as empty
• publish seed project to https://dotnet.github.io/docfx/seed

fixes #8339

[codecov]

Codecov Report

Patch coverage: 63.63% and project coverage change: -0.01 ⚠️

Comparison is base (6d80b63) 76.92% compared to head (73428b9) 76.91%.

❗ Current head 73428b9 differs from pull request most recent head 8d1036f. Consider uploading reports for the commit 8d1036f to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8727      +/-   ##
==========================================
- Coverage   76.92%   76.91%   -0.01%     
==========================================
  Files         606      606              
  Lines       25057    25064       +7     
==========================================
+ Hits        19275    19279       +4     
- Misses       5782     5785       +3     

Impacted Files	Coverage Δ
src/Microsoft.DocAsCode.App/RunPdf.cs	0.00% <0.00%> (ø)
src/docfx/Models/InitCommand.cs	0.00% <0.00%> (ø)
.../Microsoft.DocAsCode.App/Config/BuildJsonConfig.cs	86.11% <100.00%> (+0.39%)	⬆️
src/Microsoft.DocAsCode.App/RunBuild.cs	65.21% <100.00%> (+3.31%)	⬆️
src/docfx/Models/DefaultCommand.cs	91.42% <100.00%> (+0.51%)	⬆️

... and 1 file with indirect coverage changes

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