Add documentation index and metadata/errors reference

[asukhodko]

Motivation

• Provide a single entry point for the docs with a navigational map to make discovery easier.
• Surface a concise metadata reference describing Chunk fields and key chunk.metadata entries (e.g. header_path, content_type, strategy, overlaps, chunk_id, line ranges).
• Offer a short troubleshooting guide covering primary exception types (ChunkanaError, ValidationError, HierarchicalInvariantError, ConfigurationError) and practical tips.
• Update existing entry points so README and Overview point to the new pages.

Description

• Add docs/index.md as a documentation index and navigation hub with short summaries.
• Add docs/metadata.md documenting Chunk top-level fields and important chunk.metadata keys and overlap semantics.
• Add docs/errors.md with explanations and troubleshooting tips for the main Chunkana exceptions.
• Update README.md and docs/overview.md to link to the new docs/index.md, docs/metadata.md, and docs/errors.md pages.

Testing

• No automated tests were run because these are documentation-only changes.

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Codex Task