Add Pixeltable Document Store integration

[pierrebrunelle]

Summary

Adds a community integration page for haystack-pixeltable — a Haystack Document Store and Retriever backed by Pixeltable.

PyPI: https://pypi.org/project/haystack-pixeltable/
Repo: https://github.com/pixeltable/haystack-pixeltable

Components

• PixeltableDocumentStore — full Haystack DocumentStore protocol (write, delete, filter, count, serialization)
• PixeltableRetriever — vector similarity search with top-k and metadata filtering

What makes this integration different

Pixeltable is open-source multimodal data infrastructure. Unlike standalone vector databases, it provides:

• Computed columns — declaratively define transforms (embeddings, LLM calls, image processing) that auto-run on every insert
• Version history — built-in time-travel queries over your document store
• Multimodal types — native Image, Video, Audio, Document column types alongside text and embeddings
• .table escape hatch — break out of the Haystack interface to use Pixeltable's full API (add computed columns, query versions, multimodal operations)

Checklist

• .md file in integrations/ with required frontmatter
• Installation instructions
• Component list
• Runnable code examples (Document Store, Retriever, Pipeline, Filtering)
• License information
• pypi and repo fields populated