Feature Request: Add Support for Mermaid Diagram Rendering in PDF Export

[dafengqixi1026]

Description:
Currently, when converting Markdown to PDF, Mermaid code blocks are not rendered as diagrams but instead appear as raw code. This limits the usability of the tool for documentation that relies on diagrams generated from Mermaid.

Expected Behavior:
Mermaid code blocks (e.g., ````mermaid graph TD; A-->B;`) should be rendered as diagrams in the resulting PDF, similar to how they appear in GitHub Markdown previews or other supported viewers.