Adds --keyword argument to filter environments in list_envs.py

[louislelay]

Description

This PR introduces an optional --keyword <search_term> argument to the list_envs.py script. It allows filtering the list of environments by keyword, making it easier to quickly find relevant tasks.

Type of change

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

Screenshots

Searching for all Franka envs:

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

[ooctipus]

This is a nice feature!!! Thank you @louislelay

[louislelay]

Thanks @ooctipus! :)