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:

https://github.com/revolter/TestIssueTemplates/issues/1

[github-actions[bot]]

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-githubs-form-schema.md	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.18 3.17 3.16 3.15 3.14

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

🤖 This comment is automatically generated.

[Sharra-writes]

@revolter I apologize for the very late reply. Between a cold and the American holiday last week, I am trying very hard to catch up with everything. I'm not at all an expert on this, so can you explain to me what you're trying to do and what result you're getting? If needed, I can take it to the appropriate team, but only if I can articulate the question. 😅