docs: make it obvious how to open window with specific resolution or fullscreen

[emidoots]

[emidoots]

I think this should be fixed by more general API docs (autodoc for our projects), so deferring to v0.3

For anyone who stumbles across this issue: you can pass options like:

core.init(.{ .display_mode = .fullscreen });

core.init(.{ .size = .{ .width = 1920, .height = 1080 } });

or via the core.setDisplayMode and core.setSize methods.