Improve README with expanded usage, feature highlights, and API surface

[asukhodko]

Motivation

• Make the top-level README.md more useful for new users by surfacing key capabilities and common workflows.
• Document newer features present in the codebase such as code-context binding, adaptive sizing, table grouping and Obsidian block-id cleanup.
• Provide quick examples for content analysis, metrics and advanced configuration to reduce friction for integrators.
• List the primary convenience API functions so consumers can discover programmatic entry points quickly.

Description

• Expanded Quick Start with a new What You Get section summarizing structure-safety, metadata, strategies, hierarchy and streaming.
• Added feature highlights including code-context binding, adaptive sizing, table grouping, and strip_obsidian_block_ids and linked to the relevant docs.
• Included examples for content analysis and metrics using analyze_markdown and chunk_with_metrics, plus an advanced configuration snippet demonstrating ChunkConfig, AdaptiveSizeConfig, and TableGroupingConfig.
• Added a Core API Surface list with primary convenience functions such as chunk_markdown, chunk_hierarchical, chunk_file / chunk_file_streaming, analyze_markdown, chunk_with_metrics, and iter_chunks in README.md.

Testing

• Docs-only change: no automated tests were executed as part of this update.
• File modified: README.md (documentation only).
• No runtime behavior changed; normal CI test suite expected to pass for repository (not run here).
• Commit made and recorded locally as part of the change.

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Codex Task