Fix namespace and class links in documentation references that use na…

…mespace `gz` (#2385)

The `gz` namespace is not recognized by doxygen enough for it to
autogenerate links to inner namespaces and classes. For example, `\ref
gz::sim::systems` or simply `gz::sim::systems` don't link to the
namespace documentation. You'd have to use `ignition::gazebo::systems` instead. I was hoping there
was a more clever solution that would allow all `gz::` refs to work
automatically, but I was not able to find one, so this is a compromise
to fix the landing page for ign-gazebo6 (https://gazebosim.org/api/sim/6)

Signed-off-by: Addisu Z. Taddese <addisu@openrobotics.org>

api.md.in

@@ -5,10 +5,10 @@ designed to rapidly develop robot and simulation applications.
 
 **Useful links**
 
-1. gz::sim::components : List of built-in Component types. Components represent data, such as position information, that can be added to an Entity.
-2. gz::sim::systems : List of available Systems. A System operates on Entities that have a specific set of Components.
-3. gz::sim::events : List of simulation events. See the
-   gz::sim::EventManager for details about events and how to use them.
+1. [gz::sim::components](\ref ignition::gazebo::components) : List of built-in Component types. Components represent data, such as position information, that can be added to an Entity.
+2. [gz::sim::systems](\ref ignition::gazebo::systems) : List of available Systems. A System operates on Entities that have a specific set of Components.
+3. [gz::sim::events](\ref ignition::gazebo::events) : List of simulation events. See the
+   [gz::sim::EventManager](\ref ignition::gazebo::EventManager) for details about events and how to use them.
 
 ## License