Allow in-source builds of ERT in sibling directory

The source code of ERT is located in a devel/ subdirectory, but it is
not usual (?) to have this structure in a separate build directory as
well. If you however build everything in the same directory as the
source code, then this sub-directory needs to be accounted for.
Use relative path to link style.css correctly from website

If an absolute path is specified, this will be embedded into the
generated files, which is of course not correct when put on another
host.
Reinstate figure directory for documentation

The directory must be specified with full path to refer to the source
directory. The images will not be referred to by this path, but rather
copied to the output directory (as of Doxygen 1.7.6.1 at least), unlike
the stylesheet (consistency is so last decade).