Write migration guide to reposilite

[tkent]

Since Archiva development continues at a snail pace, I think it's time to slowly sunset this repo. A great self-hosted alternative has shown up reposilite and seems to have great community support. If I'm contributing to open source in this area, my time is better spent helping there.

Before winding this down, we should add a clear migration guide from Archiva to Reposilite. And, we should give folks lots of time to move off of this image and make the switch. I know many small shops have been using this image for 8+ years now.