chore: Fix exception thrown on TOC loading internally

[filzrev]

This PR is intended to fix internal exception that occurred when deserializing toc.yml file.

Current docfx implementation use YamlDeserializerWithFallback class to deserialize toc.yml file.
And following operation executed.

1. Try to deserialize yml as List<TocItemViewModel>.
2. If YamlException is thrown. then deserialize as TocItemViewModel.

---

This PR intended to fix following problems.

• On current implementation try-catch logics are used when deserializing TocItemViewModel as List<TocItemViewModel> .
  -> Exceptions should not be thrown in the normal execution flow.
• When deserializing as TocItemViewModel. It requires to open safe file twice.

[codecov]

Codecov Report

Attention: Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.81%. Comparing base (fe673ec) to head (9d56541).
Report is 211 commits behind head on main.

Files	Patch %	Lines
src/Docfx.Build/TableOfContents/TocHelper.cs	86.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9974      +/-   ##
==========================================
+ Coverage   74.31%   78.81%   +4.49%     
==========================================
  Files         536      539       +3     
  Lines       23189    23440     +251     
  Branches     4056     4063       +7     
==========================================
+ Hits        17234    18474    +1240     
+ Misses       4853     3822    -1031     
- Partials     1102     1144      +42     

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[yufeih]

This looks a bit complicated. For handling YAML, I'm leaning towards we move to a future where YAML are parsed into a JSON node tree, and then deserialized to objects with standard JsonConverter.

Similar to https://github.com/dotnet/docfx/blob/v3/src/docfx/lib/json/YamlUtility.cs#L112 but with System.Text.Json.

[filzrev]

This looks a bit complicated.

Remove stream relating unwind logics.
Instead, Read entire YAML text first, and then determine TOC type by YAML Parser.