Add WIP Crashlytics guide

src/content/docs/guides/crash-reporting/crashlytics.mdx

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+---
+title: Crashlytics Integration
+description: Integrate Shorebird into your Crashlytics crash reporting
+sidebar:
+  label: Crashlytics
+  order: 2
+---
+
+If you're using Crashlytics for crash reporting, it will work out-of-the-box
+with Shorebird releases and patches. However, if you have multiple patches, it
+can be unclear which patch caused the crash. This document shows how you can use
+Crashlytics TODO to differentiate between patches.
+
+## Add the `shorebird_code_push` package to your project.
+
+[shorebird_code_push](https://pub.dev/packages/shorebird_code_push) is available
+on pub.dev and lets you programmatically determine your app's current patch
+number. To add it to your project, follow the instructions on the package's
+pub.dev page.
+
+## Configure Crashlytics