Add OpenCelliD data source

[yuiseki]

• Close: OpenCelliD (opencellid.org) #39

Adds a new YAML file to describe the OpenCelliD data source in the repository.

• Introduces lib/data/opencellid.org/index.yaml with metadata including data_id, license, attributions, description, data_format, file_format, file_size, and url.
• Specifies the OpenCelliD data source with a data_id of opencellid_full.
• Sets the license to CC-BY-SA-4.0 and includes attributions to OpenCelliD and its URL.
• Describes the OpenCelliD data and its uses in the description field.
• Defines the data format as csv and the file format as gzipped_csv.
• Notes the file size as 105MB.
• Provides a download URL with a placeholder for the API key.

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