Add coverage of Features to Labels and Shapes guides

[psobolewskiPhD]

🧰 Task

The Points layer guide has a short section on features:
https://napari.org/stable/howtos/layers/points.html#using-the-points-features-table
Features can also be used for Labels and Shapes, but the respective layer guides don't cover this.

[psobolewskiPhD]

In addition to the copy in the Points layer guide, there is an example script for labels and shapes:

• https://napari.org/stable/gallery/add_labels_with_features.html#sphx-glr-gallery-add-labels-with-features-py
• https://napari.org/stable/gallery/add_points_on_nD_shapes.html#sphx-glr-gallery-add-points-on-nd-shapes-py

Writing up a brief description in one or both of these layer guides and linking to the respective example would be enough to close this issue.