Add docs for thick slicing

[brisvag]

References and relevant issues

• closes Add use info for both GUI and programmatic thick slicing controls #791
• depends on Qt controls for thick slicing napari#6146

Description

We finally have decent GUI and programmatic access to thick slicing. This PR adds some info about it in the viewer tutorial.

[brisvag]

Uh, it seems that the screenshots generated via code cells are not rendered in the CI's rendered docs. Is that normal?

[TimMonko]

Uh, it seems that the screenshots generated via code cells are not rendered in the CI's rendered docs. Is that normal?

It is now! Our CI is now like 3 minutes on here. See napari.org/dev/developers/contributing/documentation/index.html#contributing-to-the-napari-documentation-without-a-local-setup
You can ping the bot to run docs code with @napari-bot make docs. I'm going to ping a full build though just for fun

@napari-bot make html

[TimMonko]

@psobolewskiPhD make html gave an error https://github.com/napari/docs/actions/runs/16461947669
@napari-bot make docs

[psobolewskiPhD]

I wonder if it's because you have the command twice?

@napari-bot make html

[psobolewskiPhD]

Looks to be the case. My comment above with text and a single command worked:
https://github.com/napari/docs/actions/runs/16532997545?pr=794+%28comment%29

I can see that with two commands in one comment things get confusing. So let's just not do that 😅

[psobolewskiPhD]

Suggested change

While by default napari will only show data located *exactly* at this position, it is possible to visualize data from a thicker dimensional slice by modifying `viewer.dims.margin_left` and `viewer.dims.margin_right`, or `viewer.dims.thickness` to change both symmetrically (or the corresponding `*_step` properties to work in "slider coordinates").

By default napari will only show a slice of the data: that which is located *exactly* at this position. However, it is possible to visualize data from a thicker dimensional slice by modifying `viewer.dims.thickness`. This will use each layer's `projection_mode` to visualize the space around `viewer.dims.point`. Alternately, you can set `viewer.dims.margin_left` and `viewer.dims.margin_right` to explicitly set the range of data around `viewer.dims.point` to be projected. Finally, you can use the corresponding `margin_left_step` and `margin_right_step` properties to work in "slider coordinates".

I found the original very dense and hard to understand. I think this is clearer? But maybe no longer correct 😅

[psobolewskiPhD]

With my suggestion above, this might be removed.

Suggested change

When margins are non-zero, data is included from before (left) or after (right) the `viewer.dims.point` location, and for each layer it will be projected onto the visible space based on the `projection_mode` of that layer:

[psobolewskiPhD]

If/once we get the Features or Highlights page going, this is a great thing to break out to that page, instead of making the Viewer tutorial even more complex.