[PWAs] Performance guide for PWAs

[a2sheppy]

Part of the PWA developer guide. Provides information about performance, with a focus on tips of special note for PWA developers but with links to other existing performance documentation.

Most of this will be quick explanations of how the API is useful, anything particular to PWAs, etc, with links to content elsewhere for the details and examples, except where a PWA specific example is justified or neeeded.

Contents

• Introduction
• Progressive loading
• Using visibility state to manage resource usage
• Using Intersection Observer API
  ** Anything else?**

Acceptance criteria

• Content written and complete
• Submitted to engineering for review