[Predictive Back] Add top-level developer documentation

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docs/foundations/PredictiveBack.md

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+<!--docs:
+title: "Predictive Back"
+layout: detail
+section: foundations
+excerpt: "Predictive Back"
+iconId: predictive_back
+path: /foundations/predictive_back/
+-->
+
+# Predictive Back
+
+Predictive Back is a navigation pattern tied to gesture navigation which shows
+the user a glimpse of where swiping back will bring them. Before completing a
+swipe, the user can decide to continue to the previous view or stay in the
+current view.
+
+In Android T (13), the OS introduced support for predictive back-to-home, which
+shows the user a preview of the home screen when swiping back to exit an app.
+
+With Android U (14), the OS adds support for in-app predictive back, which apps
+can take advantage of to show the user previous destinations when swiping back
+within the app itself.
+
+## Design & API documentation
+
+-   Material Design guidelines: Predictive Back (coming soon)
+-   [Framework & AndroidX Predictive Back developer guide](https://developer.android.com/guide/navigation/predictive-back-gesture)
+
+## Usage
+
+To opt in to predictive back, apps must:
+
+1. Migrate from the legacy back handling APIs (`Activity#onBackPressed`,
+`KeyEvent.KEYCODE_BACK`, etc.) to the more recently introduced "back callback"
+APIs (`OnBackAnimationCallback`, `OnBackPressedCallback`, etc.). This involves
+flipping the `android:enableOnBackInvokedCallback` manifest flag to `true`, and
+registering callbacks to handle back pressed on Android T and above. More
+details on this general back migration can be found at the
+[Framework & AndroidX Predictive Back developer guide](https://developer.android.com/guide/navigation/predictive-back-gesture).
+
+2. Upgrade to MDC-Android library version **1.10.0-alpha01 (coming soon)** or
+above.
+
+Once completing these steps, you will get most of the predictive back animations
+within Material Components for free on Android U devices. See the section below
+to understand which components support predictive back and to learn about
+special considerations for each component.
+
+### Predictive Back Material Components
+
+The following Material Components support predictive back behavior and
+animations:
+
+- Search bar
+- Bottom sheet
+- Side sheet (support coming soon)
+- Navigation drawer
+- Navigation bar / Bottom navigation view (support coming soon)
+- Navigation rail (support coming soon)