Recommend Obtainium (#2294)

docs/android.md

@@ -305,6 +305,26 @@ Main privacy features include:
 
 ## Obtaining Applications
 
+### Obtainium
+
+!!! recommendation
+
+    ![Obtainium logo](assets/img/android/obtainium.svg){ align=right }
+
+    **Obtainium** is an app manager which allows you to install and update apps directly from the developer's own releases page (i.e. GitHub, GitLab, the developer's website, etc.), rather than a centralized app store/repository. It supports automatic background updates on Android 12 and higher.
+
+    [:octicons-repo-16: Repository](https://github.com/ImranR98/Obtainium#readme){ .md-button .md-button--primary }
+    [:octicons-code-16:](https://github.com/ImranR98/Obtainium){ .card-link title="Source Code" }
+    [:octicons-heart-16:](https://github.com/sponsors/ImranR98){ .card-link title=Contribute }
+
+    ??? downloads
+
+        - [:simple-github: GitHub](https://github.com/ImranR98/Obtainium/releases)
+
+Obtainium allows you to download APK installer files from a wide variety of sources, and it is up to you to ensure those sources and apps are legitimate. For example, using Obtainium to install Signal from [Signal's APK landing page](https://signal.org/android/apk/) should be fine, but installing from third-party APK repositories like Aptoide or APKPure may pose additional risks.
+
+Obtainium can also be used to download apps from F-Droid repositories, and may serve as a useful alternative to the official F-Droid clients. However, we generally recommend against apps built by F-Droid or from unofficial F-Droid repositories: Read [our notes on F-Droid](#f-droid) below for more information.
+
 ### GrapheneOS App Store
 
 GrapheneOS's app store is available on [GitHub](https://github.com/GrapheneOS/Apps/releases). It supports Android 12 and above and is capable of updating itself. The app store has standalone applications built by the GrapheneOS project such as the [Auditor](https://attestation.app/), [Camera](https://github.com/GrapheneOS/Camera), and [PDF Viewer](https://github.com/GrapheneOS/PdfViewer). If you are looking for these applications, we highly recommend that you get them from GrapheneOS's app store instead of the Play Store, as the apps on their store are signed by the GrapheneOS's project own signature that Google does not have access to.

docs/tools.md

@@ -427,6 +427,7 @@ We [recommend](dns.md#recommended-providers) a number of encrypted DNS servers b
 
 <div class="grid cards" markdown>
 
+- ![Obtainium logo](assets/img/android/obtainium.svg){ .twemoji } [Obtainium (App Manager)](android.md#obtainium)
 - ![Aurora Store logo](assets/img/android/aurora-store.webp){ .twemoji } [Aurora Store (Google Play Client)](android.md#aurora-store)
 - ![Shelter logo](assets/img/android/mini/shelter.svg){ .twemoji } [Shelter (Work Profiles)](android.md#shelter)
 - ![Auditor logo](assets/img/android/auditor.svg#only-light){ .twemoji }![GrapheneOS logo](assets/img/android/auditor-dark.svg#only-dark){ .twemoji } [Auditor (Supported Devices)](android.md#auditor)

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