Fix: Remove empty sidebar entry caused by `understanding-models.md`

[zihanKuang]

This PR removes the file:

content/en/kanvas/designer/understanding-models.md

The file had its front matter commented out, resulting in an empty title and causing an invisible menu item to appear under the Exploring Designer section in the sidebar.

Before

After

• [x] Yes, I signed my commits.

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[zihanKuang]

Hi @leecalcote,

I didn't create an issue for this, because the content intended for this page is already substantially covered in the Meshery documentation here: https://docs.meshery.io/concepts/logical/models.

Furthermore, this specific file had not been modified in over a year, and its conceptual focus was not well-aligned with its previous placement under the "Exploring Designer" section.🤔

So maybe we can directly remove this page.