Adds a Franka stacking ManagerBasedRLEnv

[peterd-NV]

Description

This PR adds a new ManagerBasedRLEnv for a three cube stacking task with a Franka arm.

Docs updated to add new environment type and env tested with test_environments.py script.

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Screenshots

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