Add enable_extension tip to Troubleshooting

[Tadinu]

For beginners, typically trying to figure out why a module is not found as running a script, whilst it is clearly installed (shown by pip show) may be hard.
Would it be possible to add the tip mentioned here into Troubleshooting or Known Issues with an IsaacLab-specific note that it must be after AppLauncher instantiation?
Related: isaac-sim/IsaacSim#178
Thanks!