Update docs to link to CDN guide (#2219)

Author: hsubox76

README.md

@@ -10,7 +10,7 @@
 The Firebase JavaScript SDK implements the client-side libraries used by
 applications using Firebase services. This SDK is distributed via:
 
-- CDN (`<script src="https://www.gstatic.com/firebasejs/5.5.3/firebase.js"></script>`)
+- [CDN](https://firebase.google.com/docs/web/setup/#add-sdks-initialize)
 - [npm package](https://www.npmjs.com/package/firebase)
 - [Bower package](https://github.com/firebase/firebase-bower)
 

scripts/docgen/content-sources/js/HOME.md

@@ -2,7 +2,7 @@
 The Firebase JavaScript SDK implements the client-side libraries used by
 applications using Firebase services. This SDK is distributed via:
 
-- CDN (`<script src="https://www.gstatic.com/firebasejs/{{ web_sdk_version }}/firebase.js"></script>`)
+- [CDN](/docs/web/setup/#add-sdks-initialize)
 - [npm package](https://www.npmjs.com/package/firebase)
 - [Bower package](https://github.com/firebase/firebase-bower)
 

scripts/docgen/content-sources/node/HOME.md

@@ -2,7 +2,7 @@
 The Firebase JavaScript SDK implements the client-side libraries used by
 applications using Firebase services. This SDK is distributed via:
 
-- CDN (`<script src="https://www.gstatic.com/firebasejs/{{ web_sdk_version }}/firebase.js"></script>`)
+- [CDN](/docs/web/setup/#add-sdks-initialize)
 - [npm package](https://www.npmjs.com/package/firebase)
 - [Bower package](https://github.com/firebase/firebase-bower)