MetacatUI

We already have in the works:
- Help page for MetacatUI (User oriented)
- API documentation for DataONE API (user and developer oriented)

And now I think we should add more detailed documentation for MetacatUI (developer oriented) so that users of this free, open-source software can find instructions on how to create and configure a new theme for either their own installation of Metacat or as a search tool for their DataONE member node. The Metacat docs currently have some basic documentation on theming, but I think having some stand-alone docs will be helpful since MetacatUI may be accessed individually, such as from its GitHub repo.

The Metacat docs should also be updated to put the skins instructions in a deprecated appendix section and add a link to these new UI docs in the theming section.