Fix incorrect "Next" pagination in release notes sidebar

[giorgionaps]

Summary

The “Next” button on some release note pages pointed to the current page instead of the following document.

Root cause

The issue was caused by the sidebar structure.
In certain version categories (for example 8.8 and 8.9), the same document was defined:

As the category link (category landing page), and

Again inside the items array.

This effectively registered the same doc twice in the sidebar order. Docusaurus uses the sidebar structure to compute pagination, so this duplication caused the “Next” button to resolve to the same page.

[giorgionaps]

Should be fixed by the planned restructuring of that doc section in #7797