Avoid placing broken links to documentation on index listings

[hidmic]

The fact that we do not have infrastructure to generate API documentation for ROS2 packages yet results in most documentation links in index pages for said packages to be broken. See ros2/rclcpp#958 and ros2/ros_core_documentation#16 for existing evidence.

rosindex is currently putting together URIs with no regard for whether these actually refer to existing sites or not (e.g. here). As a short term solution, we should avoid doing so. We can test URIs during index generation OR simply not generate them for ROS2 packages at all.