docs: Improve Configuring Maintenance Windows page

[andre-lfa]

I believe we should improve the Maintenance Windows page https://cloud.docs.scylladb.com/master/manage-clusters/maintenance-window to include what exactly are the maintenance operations performed during the MW and its implications.

For instance, should we expect degraded performance, potential downtime, or different SLAs for these periods? Something with a clear description, like:

Maintenance Windows are off-peak hours windows that are used to carry out necessary maintenance on the cluster, such as adding or removing nodes and Scylla version upgrades. These tasks can cause an increase in the cluster load due to different factors, such as data streaming (node addition/removal), node down temporarily (version upgrade), so it is ideal that they are carried out outside the cluster's peak hours precisely to avoid latency spikes or performance issues and so that this extra load generated by the DB does not compete with the normal cluster load. Also, maintenance windows do not affect SLAs.

Also, originally, MWs were meant to be used for operations that will require a cluster-wide RR (e.g. upgrade). However, I can see it being used more and more for scale-out/in operations when the customer doesn't normally give us a specific time, they usually agree to do it during MWs. @d-helios @ManjotS please confirm if this really is the use of maintenance windows.

cc @noellymedina @tomer-sandler

[tomer-sandler]

Just to point out that this request was also brought by external users/customers and both them and internal consumers will benefit from having the MW documentation being more verbose and clear.

Thanks!

[tzach]

@mixellent @annastuchlik ping