Add missing validations showcase to checkboxes examples

[revolter]

Why:

To prevent incorrect interpretations of the descriptions of the YAML structure required for checkboxes (like in #41190).

Closes:

What's being changed (if available, include any code snippets, screenshots, or gifs):

All the other types both explain the validations key and then showcase it, but the checkboxes one does not showcase it.

Check off the following:

• I have reviewed my changes in staging, available via the View deployment link in this PR's timeline (this link will be available after opening the PR).

  • For content changes, you will also see an automatically generated comment with links directly to pages you've modified. The comment won't appear if your PR only edits files in the data directory.

• For content changes, I have completed the self-review checklist.

[revolter]

Although, it seems like it has no effect 🤔

For example, I have this configuration:

https://github.com/revolter/TestIssueTemplates/blob/main/.github/ISSUE_TEMPLATE/test-template.yml?plain=1

but I was able to create an issue without checking any of them:

revolter/TestIssueTemplates#1

[Copilot]

Pull Request Overview

This PR adds missing validations examples to checkbox form elements in the GitHub form schema documentation to maintain consistency with other input types and prevent misinterpretation of the YAML structure.

Key Changes:

• Added validations section to two checkbox examples in the documentation

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[Copilot]

The checkbox already has required: true at the item level (line 52). Adding a top-level validations.required may be redundant or confusing. Clarify whether both are needed or if one should be removed to avoid duplication.

Suggested change

	validations:
	required: true