docs: add Pixeltable to choosing a Document Store

[pierrebrunelle]

Summary

• Add Pixeltable to the external Document Store comparison table
• Link it under Vector Databases (external integration)
• Mirrors the live listing: https://haystack.deepset.ai/integrations/pixeltable

Notes

Pixeltable is a community Document Store (haystack-pixeltable on PyPI). This docs update follows the merged integrations PR #486.

Test plan

• Confirm Pixeltable appears in the External integrations table
• Confirm the Vector Databases bullet links to the integrations page

Made with Cursor

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