docs: add HuggingFace tokenizer-based token text splitter for RAG chunking

[kela4]

Summary

This is related documentation for this pending PR: open-webui/open-webui#24139

Adds documentation for the token_transformers text splitter mode.

Checklist

• Branch was create from fork's dev branch
• Target branch is dev because it is related to a new, unreleased feature

Changes

• env-configuration.mdx: Adds token_transformers as a valid option for RAG_TEXT_SPLITTER, and documents three new
  related env vars — RAG_TOKENIZER_MODEL, RAG_TOKENIZER_MODEL_AUTO_UPDATE and RAG_TOKENIZER_MODEL_TRUST_REMOTE_CODE.
• rag/index.md: Adds a "Text Splitter Options" section explaining when to use each mode (Character, Token/Tiktoken,
  Token/Transformers), with guidance on configuring the tokenizer model for local vs. external embedding APIs and a tip
  on setting chunk size relative to the model's maximum sequence length.

[pr-validator-bot]

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