fix: support newer MathJax paths

[Mirochill]

Summary

• derive an SVG MathJax renderer path from known and current CHTML MathJax paths
• support the MathJax v4 CHTML path used by newer Sphinx versions
• keep a warning fallback instead of raising when the configured path is unknown
• add regression coverage for v3, v4, and custom CHTML paths

Fixes #27

I opened this against main, the default branch; happy to retarget if a version branch is preferred.

Validation

• git diff --check HEAD~1..HEAD
• reviewed MathJax path handling with rg/Select-String
• Tests not run locally.