docs: fix navigation order for GROUPING SETS, CUBE, ROLLUP, and SubQuery

[KananSu]

Fixed the navigation relationship between the GROUPING SETS, CUBE, ROLLUP, and Subquery documents to ensure the correct sequence: GROUPING SETS, followed by CUBE, then ROLLUP, and finally Subquery according to 'content/postgresql/sidebar.yaml'.

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Project	Deployment	Preview	Comments	Updated (UTC)
neon-next	Ready	Preview	Comment	Oct 7, 2025 2:11pm

[philip]

Thanks @KananSu, much appreciated!

I made a couple changes due to how the site changed after this PR was created.