Move rigorous design documentation to level 3

[david-a-wheeler]

I'm all for rigorous design documentation:

  - id: OSPS-48
    maturity_level: 2
    category: Documentation
    criteria: |
      The project documentation MUST provide
      design documentation demonstrating all
      actions and actors within the system.

However, this is a challenge for many developers. The reality is that many hate writing documentation, so this kind of documentation is relatively rare. I suggest this be a level 3 requirement, not level 2, so we can focus on others issues at level 2.