Add back in the marker example

[nkoenig]

🎉 New feature

The marker example was removed for some reason. I'm adding it back in.

Checkout the README.

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1972 (a3f823b) into gz-sim7 (1eb3a95) will increase coverage by 0.10%.
The diff coverage is 76.10%.

❗ Current head a3f823b differs from pull request most recent head 4adb046. Consider uploading reports for the commit 4adb046 to get more accurate results

@@             Coverage Diff             @@
##           gz-sim7    #1972      +/-   ##
===========================================
+ Coverage    64.71%   64.81%   +0.10%     
===========================================
  Files          343      352       +9     
  Lines        27430    28364     +934     
===========================================
+ Hits         17750    18385     +635     
- Misses        9680     9979     +299     

Impacted Files	Coverage Δ
include/gz/sim/ServerConfig.hh	100.00% <ø> (ø)
include/gz/sim/Util.hh	100.00% <ø> (ø)
include/gz/sim/components/Component.hh	100.00% <ø> (ø)
include/gz/sim/detail/EntityComponentManager.hh	93.86% <ø> (ø)
src/gui/plugins/modules/EntityContextMenu.cc	19.78% <0.00%> (ø)
src/gui/plugins/spawn/Spawn.cc	9.88% <0.00%> (-0.40%)	⬇️
.../gui/plugins/transform_control/TransformControl.cc	6.12% <0.00%> (+0.01%)	⬆️
...rc/systems/ackermann_steering/AckermannSteering.hh	100.00% <ø> (ø)
src/systems/joint_controller/JointController.hh	100.00% <ø> (ø)
...int_position_controller/JointPositionController.hh	100.00% <ø> (ø)
... and 31 more

... and 1 file with indirect coverage changes