feat/docs: Add explanation for cross-repository navigation

[pjlast]

From @trly :

It appears that for cross-repository functionality to work correctly a build.gradle[.kts] file needs the following set.

plugins {
    ...
    `maven-publish`
}
...
publishing {
    publications {
        create<MavenPublication>("maven") {
            from(components["java"])
        }
    }
}

This is noted in Manual configuration: https://sourcegraph.github.io/scip-java/docs/manual-configuration.html#step-4-optional-enable-cross-repository-navigation
But it isn't stated anywhere on the 'getting started page' so if users were able to get things working with scip-java out of the box, they may not notice the non-blocking/failing error

Test plan

Doc updates.