Improve project organization, and document it better

Proposed improvements to project organization and docs.

* the 5 first patches are form, but don't change the actual rules
* 1 patch is carried from @erikh, and reflects the new docs-first proposal system
* 1abffc6 is a major upgrade of the project structure, with a strawman for a new, centralized MAINTAINERS file.

Feedback welcome!
“subssystem” → “subsystem”.