Make antora docs build work without pulling submodules

[oddhack]

Per #1180 (comment), @gpx1000 volunteered to configure cmake such that we don't need to pull the submodules just to build the docs.vulkan.org samples component. Creating a tracking issue for that.

[SaschaWillems]

Would also be nice if we could somehow decouple the antora docs build from the actual samples build. For a recent PR I had to add some Linux libraries to get samples to build and had to do that in three different CI files, including the one for the antora docs build.