Document BIDS data format

We should document the BIDS format and how to bring historical data to BIDS. We should focus on the desired output (BIDS+) and any automation we can currently provide.

I propose the following:

• Write the intro to BIDS and this documentation under ./format/index.md
• Split the conversion into parts:
  • Data types ./format/datatypes/index.md
    • MRI ./format/datatypes/mri.md
  • Participants ./format/participants/index.md
  • Events ./format/events/index.md
  • Dataset information ./format/dataset/index.md