Add digit locomotion examples

[lgulich]

Description

Add an example to train a locomotion and loco-manipulation controller for digit. This also serves as an example on how to train a robot with closed loops.

Type of change

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

Screenshots

Checklist

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

[Mayankm96]

Added a few comments. Would be good to also supplement the docs with images from each task :)

[dementrock]

For this to serve as a more helpful example, would it be possible to also explain how the usd file was created?