docs: add Databento connector guide

[kafka1991]

Replace the Databento Python tutorial with a guide to the standalone Databento connector. Cover source and Docker setup, Live ingestion, Historical backfills, field mapping, and production operations, including checkpoint recovery and backpressure.

Move Databento into the Connectors sidebar category, update the connection and integration overviews, and add the Databento logo.

Validation: npm run build passed with strict link and anchor checks; the Databento guide passes Prettier.

[github-actions]

🚀 Build success!

Latest successful preview: https://preview-562--questdb-documentation.netlify.app/docs/

Commit SHA: e40bf73

📦 Build generates a preview & updates the link on each commit.

[github-actions]

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A component in src/components/ or src/theme/ was modified in this pull request.

We are creating markdown correspondents of every path (e.g. questdb.com/docs/quick-start/ → questdb.com/docs/quick-start.md) for LLM consumption.
If the component usage is shadowing useful content, it may need a converter for markdown output in convert-components.js.

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