Update the complete guide -> scss

[yl-endress]

Hello,

the line

// Import the IgniteUI themes library first
@import '~igniteui-angular/lib/core/styles/themes/index';

is deprecated and doesn't work with ignite ui version 15+

https://github.com/IgniteUI/igniteui-docfx/blob/b0d0b915789f775ab69ac2a2a111137de297c027/en/components/general/angular-grid-overview-guide.md?plain=1#L212C9-L212C58