docs: the installation page said binary packages do not exist yet

They have since 0.3.0, and the README already tells the reader to apt install
the gateway. The page now leads with the apt path and names the three packages
that genuinely are not in the index: opcua, graph_watchdog and fault_detection.

Author: bburda

docs/installation.rst

@@ -41,10 +41,25 @@ for your distribution:
    vendored copy as a fallback - no manual installation is needed. The build system
    automatically uses the vendored header when the system package is insufficient.
 
+Installation from Binary Packages
+---------------------------------
+
+ros2_medkit is in the ROS index, so most packages install with ``apt``:
+
+.. code-block:: bash
+
+   sudo apt install ros-jazzy-ros2-medkit-gateway   # or ros-humble- / ros-lyrical-
+   ros2 launch ros2_medkit_gateway bringup.launch.py
+
+Three packages are not in the index yet and have to be built from source:
+``ros2_medkit_opcua``, which is waiting on open62541pp being vendored,
+``ros2_medkit_graph_watchdog``, and ``ros2_medkit_fault_detection``.
+
 Installation from Source
 ------------------------
 
-ros2_medkit is currently distributed as source code. Binary packages will be available in future releases.
+Build from source to get every package, to run against an unreleased version, or
+to develop against the tree.
 
 1. **Create a workspace**