docs: update SDXL quickstart validation notes

[bghira]

This pull request updates the SDXL quickstart documentation in multiple languages to add explanations for new validation pipeline options. The main addition is documentation for the sdxl_validation_pipeline_mode variable, describing how to use different validation modes and how to override the peer stage checkpoint when training only one stage.

Documentation updates for SDXL validation:

• Added explanation for the sdxl_validation_pipeline_mode variable, including usage of trained-stage and full-pipeline modes, and details on how stage 1 and stage 2 interact during validation. [1] [2] [3] [4] [5] [6]
• Documented the ability to override the default base/refiner checkpoint for the peer stage using sdxl_validation_stage1_model and sdxl_validation_stage2_model when training only one stage. [1] [2] [3] [4] [5] [6]