[Website]: Document that our Elastic Agent docker images always run as unprivileged

### Type of issue

Missing information

### What documentation page is affected

https://www.elastic.co/docs/reference/fleet/elastic-agent-unprivileged

### What happened?

Elastic Agent docker images always run as unprivileged.
This is normal as running a container as root is typically an anti-pattern.

It is quite common to see a warning in the Fleet UI when running the system integration inside a docker container.
We should tell the users what to do.

### Additional info

I think the content should be contributed by the Elastic Agent team.