Fix remove to legacy in docs

[h812755591]

just one file docs/jupyter/t_geometry/pointcloud.ipynb
1,
remove pointcloud.to_legacy method
2,
Conversion between tensor and legacy point cloud ===>Conversion between tensor and point cloud
PointCloud can be converted to/from legacy open3d.geometry.PointCloud.====>PointCloud can be converted to Tensor

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[review-notebook-app]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[h812755591]

This fixes outdated documentation: to_legacy() is no longer part of the Tensor PointCloud API.
The updated wording reflects the current conversion behavior

[h812755591]

This is a documentation-only change with no user-facing behavior change,
so I did not update the CHANGELOG.

[ssheorey]

Hi @h812755591 thanks for submitting this PR. However, we don't have support for tensor point clouds in draw_geometries(). You have to use the new draw() function, which supports both legacy and tensor point clouds.