Load Google Fonts non-blocking on the demo page

[julioalucero]

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Description:

Follow-up to the font work merged in #36, mirroring the non-blocking part of ombulabs/styleguide#70. The Google Fonts request was still render-blocking.

• Made the demo page's Google Fonts link non-blocking (media="print" + onload="this.media='all'" swap) with a <noscript> fallback in index.html
• No FontAwesome changes — this repo doesn't use it (unlike the ombulabs styleguide), so the $fa-font-display part of OMBU-793: Set FontAwesome font-display to swap and load fonts non-blocking ombulabs/styleguide#70 doesn't apply here

QA

1. Open index.html, DevTools => Network => filter by fonts: the css2 request loads non-blocking and doesn't hold up first paint.
2. Headings/code render in Oxygen / Source Code Pro once the font loads.
3. Disable JS, the <noscript> fallback still loads the fonts.

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