Implement persistent underline for top-level site section in left side navigation

[kennethyyoung]

Resolves #1303

Changes:
Implements a persistent underline for top-level navigation items (e.g., Reference, Examples, Tutorials). The underline remains active both on the top-level section page and while navigating through its sub-sections.

• Added a new variable currentPath, derived from pathMinusLocal, in index.astro
• Used .startsWith() to allow the underline to persist when the current path is a sub-section
• Apply "current" to MainNavLinks to trigger the underline style
• We are using the same linklabel style used in JumpToLinks.tsx. Updates to the underline style will change both the underline for top-level navigation and sub-section links.

PR Checklist

• npm run lint passes
• [Inline documentation] is included / updated
• [Unit tests] are included/updated

Collaborative contribution from:
@kennethyyoung, @mikeng07, and @aaronalaman as part of the April 2026 CodeDay Microinternship.

[ksen0]

Great work, team, thanks! I appreciate the compactness of the fix. I have tested locally and the materials here work:

• Contributor docs ✔️
• Examples ✔️
• Tutorials ✔️
• Reference ✔️

However there is additional nesting in Community and About that loses the info:

• /libraries/ should have Community underlined (also affects events)
• /code-of-conduct/ should have About underlined (also affects access statement)
• /education-resources/ should have Tutorials underlined (exception: if this is difficult to do or requires more than ~3 lines of code, I think this can be omitted. There is some ongoing discussion on moving this anyway, so please feel free to focus on the other parts if this requires too much of a workaround.)

cc @doradocodes @Nwakaego-Ego please feel free to test this out or share your thoughts too