Navigation Submenu Block: Add example preview

[himanshupathak95]

Part of: #64707

What?

Add translatable example preview text in the Navigation Submenu block.

Testing Instructions

• Navigate to a page or post editor
• Click the inserter button (+)
• Search for "Navigation Submenu"
• Verify the block preview appears in the inserter with translatable text

Screenshot

[github-actions]

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Co-authored-by: himanshupathak95 <abcd95@git.wordpress.org>
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[himanshupathak95]

The E2E Playwright - 2 test is passing locally but failing here, rebasing again to re-run the jobs.

[himanshupathak95]

Hi @carolinan, Thanks for approving the PR.

The tests are now passing since they were fixed by #68872. This PR is now ready to be shipped 🚀