feat: add publications page link to navbar

[man080107]

Summary

This PR fixes broken internal navigation links caused by missing trailing slashes on GitHub Pages.

Some pages (e.g. /documentation, /gsoc) were being linked without a trailing slash, while the actual pages are generated at /path/. GitHub Pages does not automatically redirect these URLs, resulting in 404 errors.

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What was happening

• Pages exist at /documentation/, /gsoc/, etc.
• Navbar links pointed to /documentation, /gsoc
• Visiting the non-slash URLs resulted in a 404 page
• Browsers often autocomplete to the non-slash version, making this issue frequent

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What this PR changes

• Standardizes internal navbar links to always use trailing slashes:
  • /documentation/
  • /package-documentation/develop/
  • /tutorials/
  • /gsoc/
• Aligns link usage with Docusaurus and GitHub Pages routing behavior

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Why this matters

• Prevents 404 errors for valid pages
• Improves navigation reliability
• Avoids confusion caused by browser URL autocompletion
• Addresses the root cause discussed in issue Broken doc links when trailing slash #136 and related link issues

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How to test

1. Visit https://pecanproject.github.io/
2. Use the navbar to navigate to:
   • Documentation → Documentation
   • Documentation → Package Documentation
   • GSoC → Contributor Guidance
3. Confirm that all links load correctly without 404 errors

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Notes

This PR focuses on fixing existing broken links and does not add automated link checking or redirects, which could be handled in a follow-up issue.

[dlebauer]

Thank you for solving this! I'll merge. I'll delete GSOC.html since it seems no longer needed.

Do you have any idea why these links were working until recently?

[man080107]

Thanks for merging! I’m not entirely sure what changed recently, but my guess is that it’s related to how GitHub Pages handles URL normalization — it doesn’t automatically redirect non-trailing-slash paths. It’s possible the links appeared to work earlier due to browser caching, older build artifacts, or a change in the Docusaurus build output or deployment process.