feat: add structured logging across pipeline stages

[Ahaway1st]

Description

Add structured logging across pipeline ingest, query, and retriever stages so users can observe document processing, retrieval, generation timings, and empty-ingest conditions.

Closes #41

Changes

• Add a NullHandler to the package-level ragframework logger.
• Add INFO summaries for ingest and query stages, including retrieval, embedding, and generation timings.
• Add a WARNING when a source produces no chunks.
• Add DEBUG logging for count and embedding dimension when adding chunks to the InMemory, FAISS, and Chroma retrievers.
• Add caplog tests for pipeline, retriever, and package logging behavior.
• Document how to enable logging and update the changelog.

Type of change

• [ ] Bug fix
• [ x ] New feature / integration
• [ x ] Documentation update
• [ ] Refactor / code quality

Checklist

• [ x ] pytest tests/ -v passes locally
• [ x ] ruff check ragframework/ passes
• [ x ] mypy ragframework/ passes
• [ x ] New or updated tests cover the changes
• [ x ] Optional dependencies are guarded with a helpful import error
• [ x ] Docstrings updated where applicable
• [ x ] CHANGELOG.md updated under [Unreleased]

[adaumsilva]

Good job @Ahaway1st Thanks for contributing. PR Merged. Remember to star the repo if you didn't yet.