Move the top-level user-guide.adoc and developer-guide.adoc to their
respective source directores. This is in preparation for a future
toolchain revamp.
Move the wsug_graphics directory to wsug_src/images and wsdg_graphics
directory to wsdg_src/images. Copy common_graphics/* to the each images
directory and remove common_graphics. We only have five admonition
graphics; duplicating them lets us remove some build config overhead.
Rename wsluarm.adoc to wsdg_src/wsdg_lua_support.adoc.
Remove a dummy file.
Tested visually and by enabling `--failure-level=WARN`.
