chore: Modify logics converting xml doc comment to markdown

[filzrev]

This PR intended to fix issue #10553.

Background

Markdown can mix HTML Block inside markdown content.
When mix HTML Block inside markdown.
It need to add Empty Line between HTML Element and markdown content to parse HTML Block correctly.
(CommonMark Spec

What's changed in this PR

1. XmlComment.Extensions.cs

Add partial class that definition of GetMarkdownText method and related helper methods/fields.
GetMarkdownText method returns markdown that insert EmptyLine before/after HTMLBlock when required.

2. XmlComment.cs

• Replace XPathNavigator to use XDocument instead.
• Remove temporary workaround logics to insert new line before <pre> and preserving indent.
  (These logics are moved to GetMarkdownText method)
• Cleanup existing code.

3. XmlCommentTest.*.cs

Add test patterns for XML->Markdown conversion.

Update Snapshots

Snapshot files are updated by snapshot.yml and following changes applied.

• Extra \n is added at description content. ([It's caused by new line is inserted before <para> tag

• Empty line inserted before <pre>,<ul>,<table>,<p> tag when previous node is markdown content.
  (If prev node is HTML element. empty line is not added)