[Bug] Trim of indentation on external <code>

[rapmue]

Describe the bug

When referencing code from within xmldoc comments like:

/// <summary>
/// Interface for a program processor.
/// </summary>
/// <example><code source="../../../SDK.Examples/CustomProcessorExample.cs" region="SDK_CustomProcessor" /></example>

it will be rendered with indentation, but the code has no indentation.

To Reproduce
Steps to reproduce the behavior:

1. include an example code from external source
2. the external source code should be in the right place
3. generate the metadata and build the documentation

Expected behavior
The expected behaviour is to have the indentation stay the same as in the file, respective region which is included.

Context:

• OS: Windows
• Docfx version: [e.g. 2.73.0]
• modern template
• markdig engine

Additional context

excempt of docfx.json

"template": [
          "default",
	  "modern"
    ],
    "postProcessors": [],
    "markdownEngineName": "markdig",

Originally posted by @rapmue in #6440 (comment)

[yufeih]

There seems to be a problem handing intention when the <code> tag immediately follows <example> tag. As a workaround, this would work:

<example>
  <code ... />
</example>