[Docs] Mark Rollup as Experimental instead of Beta
Original commit: elastic/x-pack-elasticsearch@75a59e1a2624bb60a960a86038f52c0b1a23aca0
[partintro]
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-beta[]
+experimental[]
Keeping historical data around for analysis is extremely useful but often avoided due to the financial cost of
archiving massive amounts of data. Retention periods are thus driven by financial realities rather than by the
