Add more information about MRU to Buckets page

In DOC-11973, I removed a line about NRU from https://docs.couchbase.com/server/current/learn/buckets-memory-and-storage/buckets.html

This ticket is to add information about MRU to that page.

Description from previous ticket:

Couchbase bucket's use an "MFU" algorithm - Most Frequently Used for determining what to eject

This docs page talks about NRU "Not recently used"

"For all bucket-types, items are selected for ejection by means of the Not Recently Used (NRU) algorithm."

We should aim to clean this up - either removing this detail or documenting in more detail what we do.

The full details are here https://github.com/couchbase/kv_engine/blob/v7.2.0/docs/HiFi_MFU_eviction_policy.md