Expand incomplete bucket docs in globals.md

What does this MR do?

doc/charts/globals.md's "Specify buckets" section listed Pages in its default bucket names table, but the --set command block below the table didn't have a matching flag for it. Pages uses a different config key (global.pages.objectStore.bucket, not global.appConfig.*.bucket), so readers had no way to find the flag for it in that section.

While fixing that, found the backup buckets (global.appConfig.backups.bucket, .tmpBucket) have the same shape of gap.

This MR removes Pages from the table (since it isn't part of the consolidated global.appConfig object storage settings the table and --set block cover) and adds one sentence after the --set block pointing to the existing "Configure GitLab Pages" section and the existing "Backups" section in doc/advanced/external-object-storage/_index.md, instead of duplicating their --set commands here.

Related issues

Closes #6577 (closed)

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