[Internal]: Clarify that maintenance windows affect Synthetics execution

### Description

In Synthetics you can configure a monitor to be attached to a maintenance window to prevent it from running during that time (e.g. nightly during website maintenance). This prevents incorrect false alerts and potentially reporting downtime when you know the site is not available.

These are configured via _Advanced setting_ for the monitor:

Or via in project monitors via:
- `maintenanceWindows (Array)` browser monitors (see [docs](https://www.elastic.co/docs/solutions/observability/synthetics/configure-projects))
- `maintenance_windows` lightweight monitors (see [docs](https://www.elastic.co/docs/solutions/observability/synthetics/configure-lightweight-monitors))

### Resources

Maintenance windows were added to Synthetics in https://github.com/elastic/kibana/pull/222174 (a year ago), but it is not mentioned in the documentation.

There is a [reference](https://www.elastic.co/docs/solutions/observability/synthetics/configure-settings#synthetics-settings-advanced-sync-interval) to the _sync interval_ but that still doesn't describe that the monitor will stop running during the maintenance window.

### Which deployment methods does this change impact?

Elastic On-Prem and Cloud (all)

### Feature differences

This feature is available in all environments (from 8.19 onwards) and is relevant for the service and private locations.

### What Elastic Stack release is this request related to?

8.19

### Serverless release

_No response_

### Collaboration model

The documentation team will create the first draft

### Point of contact.

**Main contact:**
- @paulb-elastic

**Stakeholders:**
- @wnstn
- @shahzad31

Start with the configure-projects and configure-lightweight-monitors documentation linked in the issue, then review the Advanced sync interval reference. Update the relevant Synthetics documentation to explain that monitors attached to maintenance windows stop executing, and verify the wording covers browser and lightweight monitors in Elastic On-Prem and Cloud from 8.19 onward.

Written by the indexing model from the issue text.