Fix broken GitHub Pages links across all Nixtlaverse libraries

[khuyentran1401]

Summary

This PR fixes broken external GitHub Pages links across all Nixtlaverse libraries by converting them to proper internal Mintlify links.

Changes

• 103 files updated across hierarchicalforecast, mlforecast, statsforecast, neuralforecast, and nixtla libraries
• Converted external URLs like https://Nixtla.github.io/hierarchicalforecast/src/methods.html#topdown to internal links like src/methods.html#topdown
• Preserves hash anchors for section-specific links
• Follows Mintlify best practices for internal cross-references

Impact

• Improves site navigation and user experience
• Fixes broken cross-references within documentation
• Ensures all internal links work properly in the Mintlify documentation site

Testing

• Verified 0 remaining targeted GitHub Pages URLs across all processed files
• Links follow the navigation structure defined in each library's mint.json