Remove unreferenced switching-physics-engine tutorial and fix broken … (backport #684)

[mergify]

🦟 Bug fix

Fixes #

Summary

04-switching_physics_engine is unreferenced in the tutorials page. Instead the 04_physics_engines tutorial is referenced and points to a similar tutorial in gz-sim, but the link is broken. In this change, I've removed the 04-switching_physics_engine tutorial and fixed the link in the 04_physics_engines tutorial.

Checklist

• Signed all commits for DCO
• Added tests
• 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.

---

This is an automatic backport of pull request #684 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of c5f2954 has failed:

On branch mergify/bp/gz-physics7/pr-684
Your branch is up to date with 'origin/gz-physics7'.

You are currently cherry-picking commit c5f2954.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   tutorials/04_physics_engines.md

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by them: tutorials/04-switching-physics-engines.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[azeey]

The automated backport did not include the removal of 04-switching_physics_engine.md which I think is fine since it would be breaking an existing link.