Fix namespace and class links in documentation references that use namespace gz

[azeey]

🦟 Bug fix

Summary

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)

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.