Update API ref docs content

Mention the RN tracker and update dead links to docs.snowplow.io

Author: jethron

api-docs/docs/index.md

@@ -2,21 +2,25 @@
 sidebar_position: 1
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-# JavaScript Trackers
-The Snowplow JavaScript Trackers support being used in a number of environments. There are three versions of the tracker.
+# JavaScript trackers
+The Snowplow JavaScript trackers support being used in a number of environments.
 
 ## Web tracking
 We have two flavours of web tracker: JavaScript and Browser.
 
-- JavaScript Tracker (v2 and v3) for loading via tags, by adding code snippets to your website or Tag Manager solution.
-- Browser Tracker (v3) for installation into web apps via npm. Popular when natively integrating tracking into React, Angular and Vue applications.
+- JavaScript tracker (v2, v3, and v4) for loading via tags, by adding code snippets to your website or Tag Manager solution.
+- Browser tracker (v3 and v4) for installation into web apps via npm. Popular when natively integrating tracking into React, Angular and Vue applications.
 
-Find the documentation for both in the [Web trackers](https://docs.snowplow.io/docs/collecting-data/collecting-from-own-applications/javascript-trackers/web-tracker/) section.
+From v3 onwards, the JavaScript tracker is implemented as a wrapper around the Browser tracker.
 
+Find high-level documentation for both in the [Web trackers](https://docs.snowplow.io/docs/sources/trackers/web-trackers/) section.
 
 ## Server-side tracking
-Track events in server-side Node.js environments using the Node.js Tracker (v3). Used via npm.
+Track events in server-side Node.js environments using the Node.js tracker (v3 and v4). Used via npm.
 
-Find the documentation in the [Node.js tracker](https://docs.snowplow.io/docs/collecting-data/collecting-from-own-applications/javascript-trackers/node-js-tracker) section.
+Find high-level documentation in the [Node.js tracker](https://docs.snowplow.io/docs/sources/trackers/node-js-tracker/) section.
 
+## App tracking
+Track events in React Native environments including mobile applications with the React Native tracker.
 
+Find high-level documentation in the [React Native tracker](https://docs.snowplow.io/docs/sources/trackers/react-native-tracker/) section.