Simple game engine documentation often uses wrong link format

[SaschaWillems]

All links inside the tutorial should use xref, but the new simple game engine chapters use link instead. This does not work for the Antora site build, and leads to most of the links in the new chapters being broken, as pages referenced by link are not compiled into the site.

E.g.:

link:03_performance_optimizations.adoc[Previous: Performance Optimizations]

Should be this instead:

xref:/03_performance_optimizations.adoc[Previous: Performance Optimizations]