[REQUEST]: Document Fleet auto-upgrade feature

### Description

Create documentation about auto-upgrade feature in Fleet.
Planned to be enabled from 8.19 and 9.1.

### Resources

https://github.com/elastic/ingest-dev/issues/2878

### Collaboration

The documentation team will investigate the issue and create the initial content.

### Point of contact.

**Main contact:** @juliaElastic 

**Stakeholders:** @elastic/fleet 
