feat: Basic content prefetch

[rschristian]

Simple prefetching for content in a few cases:

• header links
• sidebar links
• links within markdown content
• blog overview links

Makes for a bit snappier nav on desktop.

[rschristian]

This shouldn't be problematic, it's kept sync'd to the context value normally:

preact-www/src/lib/i18n.jsx

Lines 56 to 59 in 67554f5

	const setAndUpdateHtmlAttr = (lang) => {
	setLang(lang);
	document.documentElement.lang = lang;
	};

While components could just pass it in & we can use the context normally, it's a bit more verbose and there's no reason to prefetch anything other than the currently set language, so abstracting it away is nicer.