[Website]: Run a Fleet Server in a container environment should be improved

### Type of issue

None

### What documentation page is affected

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

### What happened?

The section on "Run Elastic Agent in a container" should be actually split in 2:
- How to run a Fleet Server
- How to run an Elastic Agent

All examples should provide how to pass CA certs for self-signed TLS setup.

### Additional info

_No response_