Add Fivetran with open-source alternatives (Airbyte, Meltano, Datanika)

Using CONTRIBUTING_TO_OSB.md, please add:

Add proprietary tool: Fivetran
- Description: Managed ELT platform that syncs data from SaaS apps and databases into cloud warehouses
- Website: https://www.fivetran.com
- Category: Business Intelligence
- Features: Managed connectors, Automatic schema drift handling, Incremental syncs, dbt transformation scheduling, Cloud warehouse destinations

Open source alternatives:
1. Airbyte - https://github.com/airbytehq/airbyte
2. Meltano - https://github.com/meltano/meltano
3. Datanika - https://github.com/datanika-io/datanika-core

Notes on each:

- **Airbyte** (https://airbyte.com) — the largest open-source ELT project and the closest connector-count match to Fivetran.
- **Meltano** (https://meltano.com, MIT) — the declarative, code-first CLI approach, built on the Singer tap/target ecosystem.
- **Datanika** (https://datanika.io, AGPL-3.0) — bundles extract+load (dlt), transformation (dbt-core) and scheduling behind one UI. Self-hosted with `docker compose up`, or the in-tree Helm chart. 36 connectors today, an order of magnitude fewer than Fivetran's catalogue, so it suits teams who want the whole pipeline in one self-hosted tool rather than the widest connector coverage.

If Business Intelligence isn't the right home for Fivetran, Database Management also fits — happy to go with whichever you prefer.

Disclosure: I work on Datanika. Airbyte and Meltano are listed first because they are the established options here and the entry would be incomplete without them.

Start with CONTRIBUTING_TO_OSB.md to find the contribution format and the catalog location. Add the Fivetran entry under the chosen category with its description, website, features, and Airbyte, Meltano, and Datanika alternatives; done means all supplied links and notes are represented consistently.

Written by the indexing model from the issue text.