Update and clarify Elastic Agent proxy docs

The Elastic Agent takes up to 3 different proxies:
- for fleet-server,
- for the output and
- a default one used for the all internet connections, more specifically to download artifacts from `artifacts.elastic.co`.

How to configure the proxies changes if the agent is fleet-managed or stand-alone. Also with the introduction of the beta Proxy feature on Fleet>Settings, it's unclear for the customer that what is shown in the docs still valid or not.

Finally the information hierarchy isn't quite clear, for example _"Set the proxy for communicating with Elasticsearch"_ seems to be under _"Set Elastic Agent proxy settings in a standalone agent policy"_. Which should not be the case. Also on _"Set the proxy for communicating with Elasticsearch"_ shows how to do so for both, the stand-alone and fleet-managed agent.

In summary this request is for improve the [Use a proxy server with Elastic Agent and Fleet](https://www.elastic.co/guide/en/fleet/8.7/fleet-agent-proxy-support.html#fleet-agent-proxy-support) page by:
- reviewing the information hierarchy
- split more clearly what applies to stand alone agent and fleet managed
- include the new beta Proxy feature on Fleet>Settings
- Clarify which components run by the agent are affected by the `HTTP_PROXY/HTTPS_PROXY/NO_PROXY` environment variables. (see https://github.com/elastic/elastic-agent/issues/2602)