Adds ability to set platform height independent of object height for terrain

[jtigue-bdai]

Description

Allow to set platform height independent of the object height.

Type of change

• New feature (non-breaking change which adds functionality)

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

Credit: @fjenelten-bdai

[ooctipus]

having consistent mesh terrain height for repeatedObjects seems to be a nice feature.
Can you remind me if without platform_height, what issue will the original implementation run into?