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

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.