docs: clarify unmounting modals with parallel routes

[adriandlam]

Clarify that adding a page.tsx or [...catchAll]/page.tsx returning null in the slot is needed for modals to unmount on client-side navigation.

See related discussion: Modals not dismissing on forward routing #50284

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[ijjk]

Allow CI Workflow Run

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Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer