asciidoctor: 1.5.6.1 -> 1.5.6.2, also add mathematical and coderay

`asciidoctor-mathematical` requires a few system dependencies to
compile, and some of these are not linked correctly into the resulting
shared object, so these are handled manually.

`asciidoctor-mathematical` and `coderay` are necessary to compile the
vulkan specification documentation.