Move dandiset.yaml to .dandi folder

Establish a canonical `.dandi` folder for all Dandisets and soft-transition `dandiset.yaml` to reside there

Rationale: no one should be human-editing the file anyway (not even that useful to read directly TBH) and upload does not accept direct modifications from it

Key problem: as we transition to more and more BIDS, we currently require `.bidsignore` to include `dandiset.yaml` otherwise, which gets to be quite annoying (though automated tools like nwb2bids do it automatically)

Future: there are also other .dandi contents we may also want to include, such as automatic client-side records from validation (for comparison to server versions)

cc @kabilar should appreciate this I think