Add docs for installing Frigate as PWA

docs/docs/configuration/mobile_app.md

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+---
+id: mobile_app
+title: Mobile App
+---
+
+Frigate supports being installed as a [Progressive Web App](https://web.dev/explore/progressive-web-apps) on Desktop, Android, and iOS.
+
+This adds features including the ability to deep link directly into the app.
+
+## Requirements
+
+In order to install Frigate as a PWA, the following requirements must be met:
+
+- Frigate must be accessed via a secure context (localhost, secure https, etc.)
+- On Android, Firefox, Chrome, Edge, Opera, and Samsung Internet Browser all support installing PWAs.
+- On iOS 16.4 and later, PWAs can be installed from the Share menu in Safari, Chrome, Edge, Firefox, and Orion.
+
+## Installation
+
+Installation varies slightly based on the device that is being used:
+
+- Desktop: Use the install button typically found in right edge of the address bar
+- Android: Use the `Install as App` button in the more options menu
+- iOS: Use the `Add to Homescreen` button in the share menu