Convert markup and build to asciidoctor

[oddhack]

asciidoc.py is long obsolete and this gets on a path to incorporate the DF spec into Antora builds.

This is working for HTML builds although many of the images are too small (probably just needs updating the image width attributes, which have changed in asciidoctor).

PDF builds are broken ATM.

To build if you do not have the Vulkan spec toolchain already installed:

scripts/runDocker
cd /spec
make html
exit