Document LTX 2.3 text encoder download option

[omribz156]

Fixes #489.

Summary

• add the Comfy-Org single-file Gemma text encoder link to the LTX-2.3 required models list
• clarify that the existing Google Gemma link requires downloading the full repository into the text encoder folder
• leave the LoRA list unchanged because the abliterated Gemma LoRA files are not required by the repository workflows

Verification

• checked the LTX-2.3 workflow JSON files for their text encoder references
• verified the Comfy-Org text encoder file under split_files/text_encoders
• git diff --check

Implemented with Codex assistance; I reviewed the source links and kept the change to the docs-only model download list.