Add docs (+ script?) on how to use deeplearning4j-ui-standalone

See: https://github.com/deeplearning4j/deeplearning4j/issues/4503#issuecomment-357901023

We have the module/JAR, but no docs. Ideally, we should have a 1-line way to pull the JAR from maven central and start it.

## Due Date
*To be completed by:* YYYY-MM-DD

## Description
*Write a short description of what needs to be done.*

## Assignees
*Please ensure you have assigned at least one person to this issue. Include any authors and reviewers required.*

Start with the deeplearning4j-ui-standalone module/JAR and review the linked Deeplearning4j issue comment for usage context. Verify how the JAR can be obtained from Maven Central and define whether a startup script is in scope; done means documentation with a concise pull-and-start command, plus the script only if confirmed.

Written by the indexing model from the issue text.