fix: xml comment whitespace handling

[yufeih]

Whitespace in C# documentation comment is significant when treated as markdown and inside <code> blocks.

Docfx trims code and top level xml comment using the following rules:

1. Trim common leading whitespaces line by line:

        void main()
        {
            return
        }
   

   becomes

    void main()
    {
        return
    }
   

2. Trim empty leading lines and trailing whitespace and new lines:

   
   
        void main()
    {
        return
    }
   
   

   becomes

        void main()
    {
        return
    }
   

Indents are added to <code> tags except for the first line, since the first new line in <pre><code> tag is new line significant:

<remarks>
    <code source="..." />
</remarks>

becomes

<remarks>
    <pre><code>void main()
    {
        return
    }</code></pre>
</remarks>

Fixes #4017
Fixes #2623
Fixes #6440
Fixes #2830
Fixes #2523

[codecov]

Codecov Report

Patch coverage: 85.50% and project coverage change: -0.08 ⚠️

Comparison is base (1125053) 74.68% compared to head (f4d9d0c) 74.61%.

❗ Current head f4d9d0c differs from pull request most recent head 8ee0a18. Consider uploading reports for the commit 8ee0a18 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8607      +/-   ##
==========================================
- Coverage   74.68%   74.61%   -0.08%     
==========================================
  Files         629      629              
  Lines       22837    22829       -8     
==========================================
- Hits        17056    17034      -22     
- Misses       5781     5795      +14     

Impacted Files	Coverage Δ
....DocAsCode.Dotnet/Visitors/SymbolVisitorAdapter.cs	93.06% <43.75%> (-2.12%)	⬇️
...c/Microsoft.DocAsCode.Dotnet/Parsers/XmlComment.cs	85.66% <97.95%> (-1.18%)	⬇️
...ocAsCode.Dotnet/Parsers/XmlCommentParserContext.cs	100.00% <100.00%> (ø)
....DocAsCode.Dotnet/Parsers/XmlCommentTransformer.cs	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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