Convert "practical guide" example to a tutorial

[wtbarnes]

The "practical guide to XRT data visualization" would be much more well suited as a tutorial rather than an example gallery entry. It is a long form walk through of how to do a sequence of tasks, with a clear beginning and end. In contrast, most example gallery entries are short how-to guides, ie they show how to quickly perform a single, specific task.

See the SunPy tutorial as an example of this type of content.