docs: link FCP audit description to lighthouse docs (#5850)

lighthouse-core/audits/metrics/first-contentful-paint.js

@@ -12,8 +12,8 @@ const UIStrings = {
   /** The name of the metric that marks the time at which the first text or image is painted by the browser. Shown to users as the label for the numeric metric value. Ideally fits within a ~40 character limit. */
   title: 'First Contentful Paint',
   /** Description of the First Contentful Paint (FCP) metric, which marks the time at which the first text or image is painted by the browser. This is displayed within a tooltip when the user hovers on the metric name to see more. No character length limits. 'Learn More' becomes link text to additional documentation. */
-  description: 'First contentful paint marks the time at which the first text or image is ' +
-      `painted. [Learn more](https://developers.google.com/web/fundamentals/performance/user-centric-performance-metrics#first_paint_and_first_contentful_paint).`,
+  description: 'First Contentful Paint marks the time at which the first text or image is ' +
+      `painted. [Learn more](https://developers.google.com/web/tools/lighthouse/audits/first-contentful-paint).`,
 };
 
 const str_ = i18n.createMessageInstanceIdFn(__filename, UIStrings);

lighthouse-core/lib/locales/en-US.json

@@ -224,7 +224,7 @@
     "description": "The name of the metric that marks the estimated time between the page receiving input (a user clicking, tapping, or typing) and the page responding. Shown to users as the label for the numeric metric value. Ideally fits within a ~40 character limit."
   },
   "lighthouse-core/audits/metrics/first-contentful-paint.js | description": {
-    "message": "First contentful paint marks the time at which the first text or image is painted. [Learn more](https://developers.google.com/web/fundamentals/performance/user-centric-performance-metrics#first_paint_and_first_contentful_paint).",
+    "message": "First Contentful Paint marks the time at which the first text or image is painted. [Learn more](https://developers.google.com/web/tools/lighthouse/audits/first-contentful-paint).",
     "description": "Description of the First Contentful Paint (FCP) metric, which marks the time at which the first text or image is painted by the browser. This is displayed within a tooltip when the user hovers on the metric name to see more. No character length limits. 'Learn More' becomes link text to additional documentation."
   },
   "lighthouse-core/audits/metrics/first-contentful-paint.js | title": {

lighthouse-core/test/results/sample_v2.json

@@ -75,7 +75,7 @@
     "first-contentful-paint": {
       "id": "first-contentful-paint",
       "title": "First Contentful Paint",
-      "description": "First contentful paint marks the time at which the first text or image is painted. [Learn more](https://developers.google.com/web/fundamentals/performance/user-centric-performance-metrics#first_paint_and_first_contentful_paint).",
+      "description": "First Contentful Paint marks the time at which the first text or image is painted. [Learn more](https://developers.google.com/web/tools/lighthouse/audits/first-contentful-paint).",
       "score": 0.51,
       "scoreDisplayMode": "numeric",
       "rawValue": 3969.135,