[REQUEST]: Update Agent proxy documentation to add default ports

### What documentation page is affected

https://www.elastic.co/guide/en/fleet/current/fleet-agent-proxy-support.html

### What change would you like to see?

Update the documentation to note:
* The default per protocol ports:
* No protocol scheme or `http://` defaults to port 80
* `https://` defaults to port 443
* `socks5://` defaults to 1080
* Prior to 8.6.0 Endpoint (for Endpoint Security / Endpoint and Cloud Security / Elastic Defend integration) defaulted to port 1080 for all protocols, explicitly setting the port in the proxy string works around this problem -- doesn't this need to be mentioned?
* That using environment variables to define proxies will not work if Elastic Defend integration is added.
* Technically with additional steps it can work for Linux and _may_ work for macOS. There is no way to make it work for Windows.

### Additional info

cc @caitlinbetz @roxana-gheorghe @bradenlpreston @nimarezainia @cmacknz