Document tetrahedralization setup for soft lift tasks (#6967)

# Description

Documents the optional tetrahedralization dependencies required by
`Isaac-Lift-Soft-Franka` and its camera variant before users launch
training from a source checkout. The environment overview now includes
both the uv command and the legacy installer command.

The note also clarifies that `Isaac-Lift-Cloth-Franka` and its camera
variant use surface deformables and do not require these dependencies.
The `deformables.py` usage examples now request the tetrahedralization
extra, and the Kit-backed uv examples also request the Isaac Sim extra.

This documentation change introduces no new dependencies.

## Type of change

- Documentation update

## Screenshots

Not applicable.

## Validation

- `uv run isaaclab -f`
- `./isaaclab.sh -p -m py_compile scripts/demos/deformables.py`
- `uv run --isolated --extra test -- sphinx-build -W --keep-going -j
auto docs tmp/docs-tetrahedralization-pr-pr-build`

## Checklist

- [x] I have read and understood the [contribution
guidelines](https://isaac-sim.github.io/IsaacLab/main/source/refs/contributing.html)
- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`uv run isaaclab -f`
- [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 (not applicable for this documentation-only change)
- [ ] I have added a changelog fragment under
`source/<pkg>/changelog.d/` for every touched package (not applicable
because no source package changed)
- [x] My name already exists in `CONTRIBUTORS.md`

Author: mmichelis

docs/source/overview/environments.rst

@@ -209,6 +209,23 @@ for the lift-cube environment:
 * |lift-cube-ik-abs-link|: Franka arm with absolute IK control
 * |lift-cube-ik-rel-link|: Franka arm with relative IK control
 
+.. note::
+
+   The ``Isaac-Lift-Soft-Franka`` task and its camera variant require automatic tetrahedralization.
+   The ``Isaac-Lift-Cloth-Franka`` task and its camera variant use surface deformables and do not
+   require these dependencies.
+   Install the optional dependencies before running the soft-body tasks from a source checkout:
+
+   .. code-block:: bash
+
+      uv sync --inexact --extra tetrahedralization
+
+   With the legacy installer:
+
+   .. code-block:: bash
+
+      ./isaaclab.sh -i tetrahedralization
+
 .. table::
     :widths: 25 30 25 20
 

scripts/demos/deformables.py

@@ -8,10 +8,13 @@
 .. code-block:: bash
 
     # Usage with default PhysX physics and default kit visualizer.
-    uv run python scripts/demos/deformables.py
+    uv run --extra isaacsim --extra tetrahedralization python scripts/demos/deformables.py
 
     # Usage with Newton VBD backend and default kit visualizer.
-    uv run python scripts/demos/deformables.py --physics newton_vbd
+    uv run --extra isaacsim --extra tetrahedralization python scripts/demos/deformables.py --physics newton_vbd
+
+    # Install the optional dependencies for the legacy launcher.
+    ./isaaclab.sh -i tetrahedralization
 
     # Usage with OvPhysX backend without a visualizer.
     ./isaaclab.sh -p scripts/demos/deformables.py --physics ovphysx