Document where SDK and NDK versions are defined and used

docs/SUMMARY.md

@@ -70,6 +70,7 @@
         - [Directory structure](dev/core/internal/directory-structure.md)
         - [Debug Pings](dev/core/internal/debug-pings.md)
         - [Implementations](dev/core/internal/implementations.md)
+        - [Android SDK/NDK versions](dev/core/internal/sdk-ndk-versions.md)
     - [Howtos](dev/howtos/index.md)
         - [Development with android-components](dev/howtos/development-with-android-components.md)
         - [Locally-published components in Fenix](dev/howtos/locally-published-components-in-fenix.md)

docs/dev/core/internal/index.md

@@ -10,3 +10,4 @@ This includes:
 * [Directory structure](directory-structure.md)
 * [Debug Pings](debug-pings.md)
 * [Implementations](implementations.md)
+* [Android SDK/NDK versions](sdk-ndk-versions.md)

docs/dev/core/internal/sdk-ndk-versions.md

@@ -0,0 +1,29 @@
+# Android SDK / NDK versions
+
+The Glean SDK implementation is currently build against the following versions:
+
+* SDK API 28
+    * Look for `android-28` in the SDK manager
+    * or install with: `sdkmanager --verbose "platforms;android-28"`
+* Android build tools 28.0.3
+    * Download link: <https://dl.google.com/android/repository/sdk-tools-linux-3859397.zip>
+* NDK r20
+    * Download link: <https://dl.google.com/android/repository/android-ndk-r20-linux-x86_64.zip>
+
+For the full setup see [Setup the Android Build Environment](../../android/setup-android-build-environment.html).
+
+The versions are defined in the following files.
+All locations need to be updated on upgrades:
+
+* Documentation
+    * this file (`docs/dev/core/internal/sdk-ndk-versions.md`)
+    * `dev/android/setup-android-build-environment.md`
+* CI configuration
+    * `.circleci/config.yml`
+        * `sdkmanager 'build-tools;21.0.0'`
+        * `image: circleci/android:api-28-ndk`
+    * `taskcluster/docker/linux/Dockerfile`.
+        * `ENV ANDROID_BUILD_TOOLS "28.0.3"`
+        * `ENV ANDROID_SDK_VERSION "3859397"`
+        * `ENV ANDROID_PLATFORM_VERSION "28"`
+        * `ENV ANDROID_NDK_VERSION "r20"`