Include README.md in docs.ros.org package doc page

[emersonknapp]

docs.ros.org/en/jazzy/p/generate_parameter_library for example.

The document generationr automatically includes a README.md from the package directory, but this repository is structured with a README.md at the top level, and no README for the subdirectories containing package.xml

Not sure what the right solution is - https://github.com/ros2/rosbag2 also has this issue

For graph-monitor we have the toplevel README only containing repository-level meta-information and links to the individual package READMEs, but this does mean that usage information is not on the GitHub landing page, it requires a clickthrough. That may not be ideal