Update "Get involved" and "Contributing" pages

Things to address 
- https://moveit.ai/about/get_involved/ is not linked from anywhere and is mostly duplicative of https://moveit.ai/documentation/contributing/
- We point to answers.ros.org instead of the new robots stackexchange
- We point to the MoveIt Discord which does not see much activity from maintainers - GH issues and discussions are more responsive