Outdated information on proxy setup

[kassybas]

This document's description of different SDKs endpoint configs seem to be outdated.

Additionally EU seems different.

• For the new Javascript SDK it should be https://api2.amplitude.com/2/httpapi
• For the new Javascript SDK in EU datacenter it should be https://api.eu.amplitude.com/2/httpapi

I could not find documentation reference to the EU host. If there is one, it'd be useful to link it so it's easier to find.