message: docs: add pwa keyword to service worker page
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diff: # Angular service worker introduction
  -Service workers augment the traditional web deployment model and empower applications to deliver a user experience with the reliability and performance on par with natively-installed code.
  +Service workers augment the traditional web deployment model and empower applications to deliver a user experience with the reliability and performance on par with natively-installed code. Adding a service worker to an Angular application is one of the steps for turning an application into a [Progressive Web App](https://developers.google.com/web/progressive-web-apps/) (also known as a PWA).
  At its simplest, a service worker is a script that runs in the web browser and manages caching for an application.
  