Improvements to the main tutorial conclusion.adoc

## Summary

A common complaint is that, after completing a basic Vulkan tutorial, the next step is not clear. I think we should try to address this.

`conclusion.adoc` currently includes **Building a Simple Game Engine** and **AI-Assisted Vulkan Development**.

## Proposed changes

### Navigation error - BUG

I cannot find in the sidebar of the web version. Maybe it is because it is missing from `antora/modules/ROOT/nav.adoc`.

### Expand current resources

- Add links to Khronos Vulkan Samples and Sascha Willems’ Vulkan samples.
- Add links to Vulkanised. We might want to recommend some beginner-focused talks.
- The introduction contains some non-Vulkan rendering-learning resources, but we might want to repeat and expand them here.

Start with conclusion.adoc and antora/modules/ROOT/nav.adoc; check how the conclusion page is represented in the web sidebar. Add the proposed Vulkan Samples, Sascha Willems, and Vulkanised resources, expand the relevant learning resources, and verify that the conclusion page appears in navigation with working links.

Written by the indexing model from the issue text.