Revamp README: add elevator pitch, TOC, schema, requirements, performance, and compatibility

[asukhodko]

Motivation

• Provide an immediate elevator pitch and one-line USP so new users understand Chunkana at a glance.
• Improve discoverability by adding a structured Table of Contents with the requested section ordering.
• Document key integration/operational details (supported Markdown constructs, requirements, output schema, performance, compatibility) for smoother adoption.
• Preserve existing examples and usage sections while reordering docs to prioritize installation/quickstart and feature overview.

Description

• Updated README.md to add an elevator pitch and a single-line USP directly under the project title.
• Reworked the Table of Contents order and inserted new sections: Supported Markdown constructs, Requirements (Python >= 3.12), Output/Metadata schema, Performance/Scalability, and Compatibility (Dify/n8n/Windmill).
• Moved and merged existing Overview/Key Features content so installation and quickstart appear before advanced usage and integrations, while keeping original examples and renderers intact.
• Added a JSON-style example of the chunk metadata shape and called out streaming APIs such as chunk_file_streaming()/chunk_stream() for large-file workflows.

Testing

• No automated tests were executed because this is a documentation-only change.
• Manual review of the updated README.md was performed to ensure section ordering and anchors are correct.
• Existing code and examples in the README were left unchanged except for repositioning and contextual additions.
• CI (if any) should be unaffected by this docs-only update and can be run normally on merge.

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Codex Task