Update README usage examples

[asukhodko]

Motivation

• Clarify and expand the README examples to demonstrate common, production-focused workflows (streaming, hierarchical retrieval, table/LaTeX handling, custom export, and RAG sizing).
• Emphasize memory-safe streaming and guidance for choosing flat_chunks vs all_chunks for indexing vs navigation.
• Show concrete configuration patterns using ChunkConfig flags like group_related_tables, preserve_latex_blocks, max_chunk_size, and overlap_size.
• Keep documentation discoverable by updating the table-of-contents entry to Usage examples.

Description

• Renamed the old Examples section to Usage examples and updated the TOC link in README.md.
• Replaced the previous example set with five focused examples: streaming via chunk_file_streaming, hierarchical mode via chunk_hierarchical and get_flat_chunks, table/LaTeX grouping using group_related_tables/preserve_latex_blocks, custom JSON export with render_json, and a RAG sizing preset using ChunkConfig.
• Added a memory note for streaming and a short sizing guide for RAG (max_chunk_size and overlap_size).
• Only README.md was modified.

Testing

• No automated tests were run because this is a documentation-only change.
• CI will run on the branch as usual when the PR is opened, but no code tests were executed as part of this edit.

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Codex Task