[Request] Document how the Elastic Agent should be deployed in a VDI environment

## Description

We added the --delay-enroll option which was documented. The main use case for this is the VDI use case where a golden image is created with the agent. There using this flag, the user avoids having the agent enroll into fleet. Once the golden image is deployed and provisioned the agent enrolls into fleet thereby obtaining a unique agent id.

https://github.com/elastic/beats/pull/27118
https://github.com/elastic/beats/pull/27118#issuecomment-901087283

## Collaboration
*(Choose the expected collaboration model and delete the others.)*

- 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)

@nimarezainia

## Suggested Target Release

best effort

## Stakeholders

@dedemorton @blakerouse