Add documentation for npm/JVM package repos
Created by: olafurpg
Fixes https://github.com/sourcegraph/sourcegraph/issues/31717
To simplify the setup, the experimental feature flags are no longer needed to create JVM/npm package code hosts.
Test plan
This PR is mostly updating documentation so there are no new tests. I manually verified that it's possible to create npm/JVM code hosts without experimental feature flags.