fix: handle MDX placeholder variables like <id> in documentation

[hkdeman]

Enhanced remarkSafeVars plugin to preserve placeholder variables that were being stripped during MDX compilation:

• Detect mdxTextExpression nodes that look like HTML tag placeholders
• Convert expressions like {id} back to format before rendering as code
• Add support for compound variables like userId, companyId
• Maintain compatibility with existing known components
• Handle both text and flow context JSX elements

This resolves the issue where routes like /share/<id>/user were being rendered as /share//user due to MDX treating as undefined variables.

Closes #39

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[jazzberry-ai]

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