Add local site generation intructions using Antora

[walcht]

I think it would help both - new contributors who don't know a lot about Antora (like me) and users who want to navigate the tutorial locally - to have straightforward instructions on how to generate the site locally.

I have copied these instructions from the Github CI/CD YAML file.

[gpx1000]

A more "special inside knowledge" way of doing it:
https://github.com/KhronosGroup/Vulkan-Site/actions/workflows/ci.yml click on "run workflow" and select the branch for Vulkan-Tutorial that you wish to have built. Then when the CI run is complete you can download the entire Vulkan documentation site and not have to install anything.
However, yes, your build instructions for Antora for local Vulkan Tutorial is accurate; just not something we do as standard given that this is part of the whole Vulkan documentation site.
I think if @SaschaWillems is fine with it, and if @marty-johnson59 is fine with it we should accept this PR. I'm personally thinking we should make a separate README for contributors or similar that has these instructions; as I don't want the end users to feel like they're missing out on not having a local site. They do, it's part of the Vulkan SDK, and the normal website is designed to run entirely locally.

[walcht]

Sure - if needed, I can redirect this into some for-contributors README.

[SaschaWillems]

We already have a CONTRIBTUTION.adic. That would be the correct place for this.

But BEFORE you do that: Have you tested this yourself? Due to cross-references one actually needs to build the whole site, so what the CI does might not suffice. And that includes building the spec which isn't trivial. As an alternative I have set up my own testbed for site builds incl. the tutorial. I could contribute that as an easy option that works out of the box.

[walcht]

I didn't know this was supposed to be part of the entire Vulkan documentation site...

Yes - I have tested the exact same commands and the site seems to be correctly locally generated.
(well, at least what I care about as a potential contributor is there: the site is rendered in the same style and I can verify asciidoc output).

Since I know very little about Antora, it would be really nice for you to add your testbed into the CONTRIBUTION file (but I can also do that in case there are any time limitations). I don't know whether it makes sense to include building the spec just for cross-references (maybe can be added as a note...)

As noted before, I only suggested this because for new contributors to the documentation who want to see how their asciidoc renders before committing, generating the site locally is helpful for that. Thanks :-)

[SaschaWillems]

If your PR works fine locally, then there's prob. no need for the testbed. So feel free to move the documentation to the contribution guide and we're good to go.