Add documentation on how to use datasets published by Transitdata

[mjaakko]

Add documentation how to use datasets published by Transitdata (e.g. HFP CSV dumps)

Documentation should contain:

• Data format
• Where the data is available
  • Authentication etc.
• How the data is published
  • e.g. how often the data is published
• Anything else that is relevant for the user of the data

Datasets to be documented:

• [x] HFP CSV dumps
• [x] GTFS-RT feeds
• [x] EKE CSV dumps
• [x] APC parquet data

[mjaakko]

Pull requests have been made to the data catalogue

[mjaakko]

Pull request created also for APC data

[mjaakko]

Documentation has been updated