Bug 1829017 - Add a synthetic document about how to build Firefox for Android. r=sylvestre

Differential Revision: https://phabricator.services.mozilla.com/D175962

docs/contributing/build/building_mobile_firefox.rst

@@ -7,9 +7,9 @@ devices, and audiences:
 -  Building **Firefox for Android** (codename: fenix). Our general-purpose
    mobile browser is split into several different artifact layers:
 
-  - `The fenix Android application <https://github.com/mozilla-mobile/firefox-android/tree/main/fenix>`_
+  - `The fenix Android application </mobile/android/fenix.html>`_
   - `The android-components Android library <https://github.com/mozilla-mobile/firefox-android/tree/main/android-components>`_
-  - `The GeckoView platform <https://firefox-source-docs.mozilla.org/mobile/android/geckoview/index.html>`_
+  - `The GeckoView platform </mobile/android/geckoview>`_
 
 -  `Firefox for iOS <https://github.com/mozilla-mobile/firefox-ios>`_,
    our general-purpose browser for iOS with desktop sync built-in.

mobile/android/docs/fenix.rst

@@ -0,0 +1,51 @@
+Building Firefox for Android
+============================
+
+First, you'll want to `set up your machine to build Firefox </setup>`_.
+Follow the instructions there, choosing "GeckoView/Firefox for Android" as
+the bootstrap option.
+
+Once you're set up and have a GeckoView build from the above, please
+continue with the following steps.
+
+1. Clone the repository and initial setup
+-----------------------------------------
+
+.. code-block:: shell
+
+    git clone https://github.com/mozilla-mobile/firefox-android
+    cd firefox-android/fenix
+    echo dependencySubstitutions.geckoviewTopsrcdir=/path/to/mozilla-central > local.properties
+
+replace `/path/to/mozilla-central` with the location of your mozilla-central/mozilla-unified source tree.
+
+2. Build
+--------
+
+.. code-block:: shell
+
+    export JAVA_HOME=$HOME/.mozbuild/jdk/jdk-17.0.6+10
+    export ANDROID_HOME=$HOME/.mozbuild/android-sdk-<os_name>
+    ./gradlew clean app:assembleDebug
+
+`<os_name>` is either `linux`, `macosx` or `windows` depending on the OS you're building from.
+
+
+For more details, check out the `more complete documentation <https://github.com/mozilla-mobile/firefox-android/tree/main/fenix>`_.
+
+3. Run
+------
+
+From the gecko working directory:
+
+.. code-block:: shell
+
+    ./mach android-emulator
+
+
+From the firefox-android working directory:
+
+.. code-block:: shell
+
+   ./gradlew :app:installFenixDebug
+   "$ANDROID_HOME/platform-tools/adb" shell am start -n org.mozilla.fenix.debug/org.mozilla.fenix.debug.App