Add PDF generation ability to Microsoft.DocAsCode.App

[dpvreony]

Greetings

• Adds the ability to target the pdf config section in docfx.json
• Adds logging around templates not being resolved, as came across an issue which had no logging and results in 'No TOC file is included, no PDF file will be generated'

partial resolution of #8872

[yufeih]

@dpvreony build failed with:

Error: /home/runner/work/docfx/docfx/test/docfx.Tests/DocsetTest.cs(198,30): error xUnit1013: Public method 'Build_With_RedirectUri_Files' on test class 'DocsetTest' should be marked as a Fact. Reduce the visibility of the method, or add a Fact attribute to the method. [/home/runner/work/docfx/docfx/test/docfx.Tests/docfx.Tests.csproj::TargetFramework=net6.0]
Error: /home/runner/work/docfx/docfx/test/docfx.Tests/DocsetTest.cs(198,30): error xUnit1013: Public method 'Build_With_RedirectUri_Files' on test class 'DocsetTest' should be marked as a Fact. Reduce the visibility of the method, or add a Fact attribute to the method. [/home/runner/work/docfx/docfx/test/docfx.Tests/docfx.Tests.csproj::TargetFramework=net8.0]
Error: /home/runner/work/docfx/docfx/test/docfx.Tests/DocsetTest.cs(198,30): error xUnit1013: Public method 'Build_With_RedirectUri_Files' on test class 'DocsetTest' should be marked as a Fact. Reduce the visibility of the method, or add a Fact attribute to the method. [/home/runner/work/docfx/docfx/test/docfx.Tests/docfx.Tests.csproj::TargetFramework=net7.0]

[yufeih]

Some tests that validate logs failed due to new warnings. We could either fix those tests or drop the new warning change.

[yufeih]

@dpvreony It seems tests are still failing.

[codecov]

Codecov Report

Patch coverage: 61.11% and no project coverage change.

Comparison is base (1aa67a6) 76.80% compared to head (a33973d) 76.80%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8939   +/-   ##
=======================================
  Coverage   76.80%   76.80%           
=======================================
  Files         592      592           
  Lines       24716    24732   +16     
=======================================
+ Hits        18983    18996   +13     
- Misses       5733     5736    +3     

Impacted Files	Coverage Δ
src/Docfx.App/Docset.cs	73.07% <53.33%> (-26.93%)	⬇️
...ine/TemplateProcessors/TemplateModelTransformer.cs	85.18% <100.00%> (+0.07%)	⬆️
...ild.Engine/TemplateProcessors/TemplateProcessor.cs	88.23% <100.00%> (+0.35%)	⬆️

... and 2 files with indirect coverage changes

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[yufeih]

Thank you @dpvreony for adding this feature and taking the time to make sure tests work!