Implement text encoding nodes:
- clip_text_encode_flux.py: Flux encoder with separate CLIP-L and T5-XXL prompts
- clip_text_encode_flux_context.py: context-based Flux encoder
- clip_text_encode_sdxl.py: SDXL encoder with CLIP-L/G prompts and aesthetic scores
- clip_text_encode_sdxl_context.py: context-based SDXL encoder
Add markdown documentation for all text encoder nodes