Metacat

Added a Sphinx theme for the documentation that mirrors the new default theme for MetacatUI.

https://projects.ecoinformatics.org/ecoinfo/issues/6075

Moving Metacat Sphinx RST documentation from docs/dev to docs/user directory.

Adding new documentation section for Metacat design documents in Sphinx/ReStructured Text format. Build this documentation using the
provided Makefile, assuming that both Sphinx and PlantUML are installed and on the local executable path by running 'make plantuml html'....