Write usage guide for the scheduled maintenance feature

[lukeheath]

Goal

User story
As a Fleet admin that wants to use the scheduled maintenance feature,
I want to read a usage guide that explains Fleet's best practice usage
so that I understand how the feature is expected to work.

Instructions for writing a usage guide are here: https://github.com/fleetdm/fleet/pull/20668/files

Context

• Requestor(s): @lukeheath

Related tickets

• This ticket is to create an official guide
• Scheduled maintenance announcement is out-of-date #20416 is to fix any text copy changes on the announcement that are out of date
• Update maintenance window graphic on fleetdm.com #20680 is to fix any image changes that are out of date

[lukeheath]

@sharon-fdm I realized we don't have a guide for the scheduled maintenance feature. We'll need to write and publish one before we release 4.55, so I'm creating this P2 release blocker to track the request. Please estimate this and assign out. We have a specific process for writing these using a template. Instructions for writing a usage guide are here: https://github.com/fleetdm/fleet/pull/20668/files. Will be in the engineering handbook soon. Thanks!