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

[yarikoptic]

This PR was initially submitted as #1861 but I made a mistake to combine it with a discussion of transformations of existing projects' layouts into such BIDS project dataset. Please refer to that PR for examples but otherwise let's concentrate here on the discussion of this specific proposed change.

• 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, derivative 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.
  • See Add DatasetType="project" and rework existing "layout" example into a proper BIDS dataset #1861 on some examples and initial discussions on possible "transformations". But this PR has merit regardless whether individual existing projects adopt it partially or fully.
• 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:

• provided more relation to existing approaches (mostly by @snastase in Add DatasetType="project" and rework existing "layout" example into a proper BIDS dataset #1861)
• crafted example for bids-examples : Add examples with DatasetType = "project" bids-examples#451
  • ensure bids-validator with modified schema passes its validation