Don't override github.external_url if its already defined

[mdedetrich]

So I came across a bug from a specific corner case when trying to integrate github ref links into one of my projects. After some debugging I found the state that causes the issue

1. Have scmInfo defined (in my case its because we use ghpages to publish the paradox site to github pages)
2. Define your own custom github.external_url in paradoxProperties. In my case this is done because we have a main branch rather than a master branch.

When I tried doing this in my project I noticed that no matter how I overrode github.external_url the setting would never apply, after looking into Paradox I found the offending code, i.e. https://github.com/lightbend/paradox/blob/master/plugin/src/main/scala/com/lightbend/paradox/sbt/ParadoxPlugin.scala#L427-L436

Basically if you have scmInfo defined then it will always override a custom defined github.external_url because the linkProperties just ignores github.external_url and returns a hardcoded github.external_url value that overrides the original.

[johanandren]

LGTM

[mdedetrich]

I added one more commit which updates the documentation with the changed behavior.