doc/rados: add osd_deep_scrub_interval setting operation

doc/rados: add osd_deep_scrub_interval setting operation

Add a second method of changing the value of osd_deep_scrub_interval to
remedy the condition indicated by the "PGs not deep-scrubbed in time"
warning.

This procedure was developed by Eugen Block, and is at the time of this
commit available on his blog at
https://heiterbiswolkig.blogs.nde.ag/2024/09/06/pgs-not-deep-scrubbed-in-time/

Co-authored-by: Eugen Block <eblock@nde.ag>
Signed-off-by: Zac Dover <zac.dover@proton.me>