[Connectors] Add support for Dropbox

Add the source and target processors to allow using Dropbox as connector.

The existing s3 and google_drive connectors could be of inspiration for it. See https://github.com/docling-project/docling-jobkit/tree/main/docling_jobkit/connectors.

The connectors can easily be used and tested via the `docling-jobkit-local` CLI. See the [docs](https://docling-project.github.io/docling/usage/jobkit/) for some example.

### Expected outcome

- The connector can be used as source and target within the `docling-jobkit-local` CLI
- Add documentation on how one can configure the connector, e.g. how to obtain the api keys, etc