add info on how to cross-reference gallery examples

[TimMonko]

References and relevant issues

Cross referencing used in napari PR#7426 and was a source of confusion for me.

Description

This documentation adds how-to cross-reference gallery examples. Info is included for both myst and rst formats and attempts to clear up what I was confused about.

[TimMonko]

How can I understand why CircleCI is failing? It builds locally for me without this issue, and build finished with problems, 1 warning. make: *** [Makefile:31: docs-build] Error 1 is not descriptive?

[psobolewskiPhD]

It's tricky especially right now with all the qt_window spam.
So the hint is that there was a warning. We set up the build to fail on warnings, so we deal with them.
So you need to search for WARNING but after the examples conversions are finished, so the proper build phase.
In this case I suspect it's this:

/home/circleci/project/docs/docs/developers/contributing/documentation/index.md:585: WARNING: 'myst' cross-reference target not found: 'gallery/add_image.html'

https://app.circleci.com/pipelines/github/napari/docs/1012/workflows/9435b88d-7504-47ad-97f4-bf432d7e967e/jobs/1003?invite=true#step-106-128127_93

[psobolewskiPhD]

Awesome! 😎