add section on dandi layout and improve related info for generating an organized dandiset #201

based on a recent discussion thread, i went to look at description of dandi layout in our docs. i couldn't seem to find a clear description of the dandi layout anywhere, even though there is mention of the tools that may get people there. also the mention of dandi organize is hidden relative to the types of data. more people are coming with multiple data types and it would be good to provide a bit of guidance around this.

even at a basic level of when to use DANDI layout or BIDS.