Metacat

Modifications to make the build more readily find locally installed versions of
sphinx-build, and to remove the unneccary 'html' directory; output of
admindoc is now written directly into build/docs.

Added links to Javadoc. Tuned the layout a bit.

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'....