[ros2] Update documentation for installation instructions and bridge examples

[AndrejOrsula]

🦟 Bug fix

Summary

Few minor changes to installation instructions and examples for ROS 2. Tested on Ubuntu Focal, with Edifice built from source (Docker).

Installation instructions:

• rosdep install complains if Ignition libraries are included in requirements
  • Skipping keys for Citadel, Dome and Edifice versions of the required libraries (I wish there was a wildcard support)
• Both README.md and ros_ign_bridge/README.md contained installation instructions
  • Removed instructions from ros_ign_bridge/README.md
  • Note: Please let me know if there was a reason behind this. If so, we can revert the change and update the instructions there so that they are correct

Examples:

• Source command of ROS 2 workspace was using invalid ROS distribution (melodic instead of foxy)
  • Fixed
• Bridge example for image transport contained a screenshot for the example using ROS 1
  • Replaced with ROS 2 alternative

Checklist

• Signed all commits for DCO
• Updated documentation (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)

Note to maintainers: Remember to use Squash-Merge

[ahcorde]

I think it makes sense to just keep the install instructions in one place, the other libraries don't have install instructions in the particular README.md.

@chapulina do you have a different opinion ?

[chapulina]

Removed instructions from ros_ign_bridge/README.md

Thanks, that was already done for ROS 1 but we never updated ROS 2.

[AndrejOrsula]

Note: Added one more commit into this PR to remove FluidPressure TODO from support matrix. Demo for it was enabled and tested in #144, but I added the commit here to avoid merge conflicts.