Remove unreferenced switching-physics-engine tutorial and fix broken …

[shameekganguly]

🦟 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)
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