doc/rados: add "pgs not deep scrubbed in time" info

doc/rados: add "pgs not deep scrubbed in time" info

Add a procedure to doc/rados/operations/health-warnings.rst that
explains how to remedy the "X PGs not deep-scrubbed in time" health
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>
(cherry picked from commit d620a51c306d1ebcdfc974dc0b85dc46cfde8d40)