[Synthetics] Document new fleet 'complete' image / integration

[andrewvc]

Description

With the synthetics browser integration coming out in 7.15.0 along with the new complete docker image we'll need to document usage with fleet in the synthetics guide. We also need a link to send people viewing the callout added https://github.com/elastic/kibana/pull/108935/files in kibana.

Collaboration

• The docs team will lead producing the content
• The product team will provide the initial content and the docs team will edit / review
• The docs team will define with product team the structure and location, and the product team will provide the initial content
• Other (please describe)

Suggested Target Release

v7.15.0

Stakeholders