feat(docs): introduce C++ documentation

[zjw1111]

Purpose

Linked issue: N/A

Introduce the C++ documentation tree migrated from the Alibaba Paimon C++ repository.

This pull request includes:

• Sphinx documentation build files under docs/.
• API reference pages, getting started pages, examples, and user guide pages under docs/source/.
• Documentation static assets under docs/source/_static/.
• A narrow .gitignore exception for the migrated docs/source/_static/prefetch.svg asset.

License handling:

• Replaced Alibaba-owned documentation headers with ASF headers.
• Preserved existing Apache Arrow and Apache Paimon attribution comments for borrowed documentation/build files.
• No LICENSE or NOTICE updates are required for this batch.

External contributor handling:

• No external contributor threshold was reached, so no Co-authored-by trailer is included.

Tests

API and Format

No API or storage format changes.

Documentation

Adds the C++ documentation tree.

Generative AI tooling

Migrated-by: OpenAI Codex

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