Build Antora docs site as part of CI

[oddhack]

Augment CI to build the Antora static site (not the full docs.vulkan.org, just the 'spec' module). This will both serve as validation before possibly affecting the full site build in the Vulkan-Site repository, and eventually enable the validation layers to link into a version of the site rather than the enormous (slow) single-page specification.

[oddhack]

Ref @spencer-lunarg @KarenGhavam-lunarG .