Add FCP link to TTFB Further Reading

Author: nucliweb

pages/Loading/TTFB.mdx

@@ -272,3 +272,4 @@ new PerformanceObserver((entryList) => {
 - [Optimize TTFB](https://web.dev/articles/optimize-ttfb) | web.dev
 - [Navigation Timing API](https://developer.mozilla.org/en-US/docs/Web/API/Navigation_timing_API) | MDN
 - [Resource Timing API](https://developer.mozilla.org/en-US/docs/Web/API/Performance_API/Resource_timing) | MDN
+- [FCP](/Loading/FCP) | First Contentful Paint — the next metric after TTFB