fix: standardise local asset URLs to use Liquid filter in head includes

[Parth-Garg2629]

Local asset references in _includes/head.html and _includes/head_print.html were using bare paths (e.g. href="css/main.css") inconsistently alongside already-correct Liquid-filtered paths (e.g. href="{{ "css/syntax.css" }}").

Bare paths silently break when Jekyll is deployed with a non-root baseurl. Wrapping all local asset paths in {{ "..." }} ensures correct URL resolution in all deployment scenarios.

Changed files:

• _includes/head.html — 10 paths standardised
• _includes/head_print.html — 19 paths standardised

No functional change for root deployments (like precice.org).

Closes #787

[Copilot]

Pull request overview

This PR standardizes local asset URL references in Jekyll head includes by wrapping bare paths in Liquid output syntax {{ "..." }}. While functionally this produces the same output as bare paths (since no filters are applied), it creates consistency with the existing pattern used for some assets like css/syntax.css. The site's flat permalink structure (all pages served from root level) means relative paths work correctly without requiring baseurl handling.

Changes:

• Wrapped 11 local asset paths in _includes/head.html in Liquid output tags (favicons, CSS files, and JavaScript files)
• Wrapped 18 local asset paths in _includes/head_print.html in Liquid output tags (favicons, CSS files, and JavaScript files)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
_includes/head.html	Standardized 11 local asset references (images, CSS, JS) to use {{ "path" }} syntax instead of bare paths
_includes/head_print.html	Standardized 18 local asset references (images, CSS, JS) to use {{ "path" }} syntax instead of bare paths

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