add working groups section and fill-in client library details

[alsora]

This PR adds:

• a section to list official WGs
• an entry list for the client library WG
• a directory where to store WG charters

[ros-discourse]

This pull request has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/next-client-library-wg-meeting-friday-20th-december-2024-8am-pt/41179/7