Add tutorial for using components in systems

[mabelzhang]

🎉 New feature

Practices:

• I tried to keep it as straight-forward as possible, only providing brief explanation when needed. This is so that tutorials are not explanations, following the Divio documentation principles.
• Links to other pages are provided when context and examples are needed, so that a user who isn’t a dev team developer can understand what we’re talking about. But, IANAD, IANAL, IANAU (I am not a user).
• For the sake of maintenance, all links are \ref links or relative links, except for links going to GitHub source code.

Summary

This PR adds two pages.

Parent page is an overview of using components in systems. It has a high-level overview of what people need to know, linking to prerequisites and resources. It then talks about the ECM object in practical terms as the gateway to access entities and components.

It then links to a list of case studies, of which there is just 1 item right now, for JointForceCmd component (child page). The assumption is that we will be adding more case studies to this list for other components.

The child page uses the simplest built-in system plugin ApplyJointForce to explain how to use the JointForceCmd component through the ECM, by finding the Joint and then modifying the component.

Test it

colcon build --merge-install --packages-select gz-sim8 --cmake-target doc

Navigate browser to build/gz-sim8/doxygen/html/usingcomponents.html (parent page) and build/gz-sim8/doxygen/html/jointforcecmdcomponent.html (child page).

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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

All modified and coverable lines are covered by tests ✅

Comparison is base (9f677bf) 65.63% compared to head (3fdb3bd) 65.61%.

Additional details and impacted files

@@             Coverage Diff             @@
##           gz-sim8    #2207      +/-   ##
===========================================
- Coverage    65.63%   65.61%   -0.02%     
===========================================
  Files          324      324              
  Lines        30925    30924       -1     
===========================================
- Hits         20298    20292       -6     
- Misses       10627    10632       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mabelzhang]

DCO was complaining because the 4 automatic signoffs via GitHub web UI used gmails. I just set it to pass since we'll squash before merging anyway.

Ready for another look.

[mabelzhang]

DCO complained again, so I fixed it for real.