adding a new property, backgroundImage, to Item

[djarecka]

see discussion in ReproNim/reproschema-py#100

[djarecka]

@yibeichan - can you check why the mardown is failing

[djarecka]

we will have to also update workflows to use updated commands from linkml

[yibeichan]

[✖] https://www.nlm.nih.gov/cde/index.html → Status: 404
[✖] https://more.metadatacenter.org/tools-training/outreach/cedar-template-model → Status: 0 Error: ESOCKETTIMEDOUT

the above are two dead links, I'll fix them

[Copilot]

Pull Request Overview

This PR adds a new backgroundImage property to the Item class in the LinkML schema to support background images for drawing activities. The change expands the schema to accommodate visual drawing interfaces that require background image support.

• Defines a new backgroundImage slot with appropriate type constraints and documentation
• Integrates the new property into the Item class structure

[Copilot]

[nitpick] The description should be more detailed and include information about expected formats, usage context, or relationship to drawing activities. Consider expanding to: 'Background image to be displayed during drawing activities. Can be specified as either an ImageObject or URI reference to an image resource.'

Suggested change

	Background image for drawing activities.
	Background image to be displayed during drawing activities. Can be specified as either
	an ImageObject or URI reference to an image resource.