This change renames Isaac Lab extensions and folders to simplify the
directory structure.

- source/extensions/omni.isaac.lab --> source/isaaclab
- source/extensions/omni.isaac.lab_assets --> source/isaaclab_assets
- source/extensions/omni.isaac.lab_tasks --> source/isaaclab_tasks
- source/apps --> apps
- source/standalone --> scripts
- source/standalone/workflows/rl_games -->
scripts/reinforcement_learning/rl_games
- source/standalone/workflows/rsl_rl -->
scripts/reinforcement_learning/rsl_rl
- source/standalone/workflows/skrl -->
scripts/reinforcement_learning/skrl
- source/standalone/workflows/sb3 --> scripts/reinforcement_learning/sb3
- source/standalone/workflows/ray --> scripts/reinforcement_learning/ray

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- New feature (non-breaking change which adds functionality)
- Breaking change (fix or feature that would cause existing
functionality to not work as expected)
- This change requires a documentation update

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) 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
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

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