avoid duplicate API documentation for ImageViewer, CollectionViewer

[grlee77]

Recent PRs have been failing on some of the CI jobs due to warnings from Sphinx about duplicate entries in the API docs for CollectionViewer.

It seems that both ImageViewer and CollectionViewer are imported under both skimage.viewer and skimage.viewer.viewers. apigen.py generates API docs for these two classes for both locations and sphinx 4.x raises a warning about this.

As a workaround, this PR removes the skimage.viewer.viewers submodule from the autogenerated API documentation so that there are no longer duplicate occurrences. I'm not sure if there is any better workaround. The warning message generated by sphinx mentions adding :noindex: to one of the instances in the .rst files, but that didn't seem applicable here since the .rst files are autogenerated by the tools/build_modref_templates.py script.

Description

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.

[grlee77]

This has resolved the CI failures that have been happening this past week

[mkcor]

Thank you so much, @grlee77! I find your workaround satisfactory; I briefly looked into the issue and couldn't find a better solution:

The warning message generated by sphinx mentions adding :noindex: to one of the instances in the .rst files, but that didn't seem applicable here since the .rst files are autogenerated by the tools/build_modref_templates.py script.

Indeed.

I guess I can merge this PR even without a second approval, since it's a CI hotfix.