Contact, wheel slip, and friction documentation

[scpeters]

🎉 New feature

Summary

This adds the following documentation of the following topics:

• Simulation of contact, slip, and friction in Gazebo
• Special considerations for modeling wheel contact / slip / friction

I'm not sure if a gz-physics tutorial is the right place for this; I'm open to moving it elsewhere. It still needs a few figures, but it collects a lot of knowledge about contact simulation in general and specifically for wheel-terrain contact. It includes both Gazebo Classic and gz-sim.

Test it

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.