[Internal]: Add maintenance window to the stack upgrade doc

### Description

Kibana [maintenance windows](https://www.elastic.co/docs/explore-analyze/alerts-cases/alerts/maintenance-windows) is a great feature. It should be included whenever users run upgrade. This is relevant to both cloud/on-prem (but not serverless) upgrades to reduce noise.

### Resources

https://www.elastic.co/docs/explore-analyze/alerts-cases/alerts/maintenance-windows

### Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

### Feature differences

- On-Prem: relevant
- Cloud Hosted: relevant
- Cloud Serverless: irrelevant

### What release is this request related to?

N/A

### Serverless release

N/A

### Collaboration model

The documentation team

### Point of contact.

**Main contact:** N/A

**Stakeholders:** N/A