[Docs] Fix 404 asset base path for previews

[CodexRaunak]

Description

Fixes the Layer5 docs 404 page asset base path so missing nested routes render with styling in production while preserving relative behavior for GitHub Pages PR previews.

Root cause

The generated 404 page emits relative asset URLs like ./scss/... and ./js/.... On missing nested production paths such as /foo/ or /foo/bar, those relative URLs resolve against the missing route depth and can load incorrectly.

Changes

• Add a 404-only runtime base path before relative assets load.
• Use / for production docs paths.
• Preserve /pr-preview/pr-N/ paths, including the GitHub Pages repository prefix, for PR previews.
• Create and insert the element via DOM APIs instead of document.write().

Validation

• Confirmed production and preview workflows use root-relative builds with GitHub Pages preview subpaths.
• Built docs with Hugo using production-style --baseURL / into /private/tmp/layer5-docs-404-review-suggestion.
• Verified generated 404.html includes the DOM-created base script before relative CSS and JS asset links, and does not use document.write for the 404 base path.

[gemini-code-assist]

Code Review

This pull request adds a script block to layouts/partials/head.html for 404 pages to dynamically set the <base> tag's href attribute when in a PR preview environment. The reviewer suggested updating the JavaScript variable declarations from var to const to align with modern standards and improve block-scoping.

[gemini-code-assist]

Using var for variable declarations is an outdated practice in modern JavaScript. It is highly recommended to use const (or let if re-assignment is needed) to ensure block-scoping, prevent accidental re-declarations, and improve overall code maintainability and readability.

Suggested change

	(function () {
	var path = window.location.pathname;
	var previewPath = path.match(/^((?:\/[^/]+)?\/pr-preview\/pr-[^/]+\/)/);
	var basePath = previewPath ? previewPath[1] : "/";
	var base = document.createElement("base");
	base.href = basePath;
	document.head.insertBefore(base, document.head.firstChild);
	})();
	(function () {
	const path = window.location.pathname;
	const previewPath = path.match(/^((?:\/[^/]+)?\/pr-preview\/pr-[^/]+\/)/);
	const basePath = previewPath ? previewPath[1] : "/";
	const base = document.createElement("base");
	base.href = basePath;
	document.head.insertBefore(base, document.head.firstChild);
	})();

[github-actions]

🚀 Preview deployment: https://layer5io.github.io/docs/pr-preview/pr-1066/

Note: Preview may take a moment (GitHub Pages deployment in progress). Please wait and refresh. Track deployment here