Add DatasetType="project" and rework existing "layout" example into a proper BIDS dataset

[yarikoptic]

• Rationale 1 (major): BIDS standard already provides reasonable structure to formalize organization of various components of a neuroscientific data project: where to place code, original (source) data, derivaitve data, README, CHANGES. Many projects (e.g. nipoppy, YODA, etc) propose similar and often might be even "inspired" templates . If we explicitly allow for BIDS standard to prescribe study level organization, IMHO it would help many people and projects decide on how to organize their studies/projects.
• Rationale 2: IMHO BIDS standard should describe only what standard prescribe and not recommend some potential "non-standardized" layouts. That is why I "reworked" that example into a legitimate BIDS dataset merely by adding dataset_description.json.

TODOs:

• [x] provide more relation to existing approaches (attn @snastase)
• [started] craft example(s) for bids-example : https://github.com/bids-standard/bids-examples/pull/451
  • [ ] ensure bids-validator with modified schema passes its validation