Put the maturity model in the handbook

Currently (as per gitlab-com/www-gitlab-com!71752 (merged)), the maturity model is in a spreadsheet: https://docs.google.com/spreadsheets/d/1i4-K__1nDpuSPnTbghHWiof7YJnlXQXA6NabCvedbwU/edit#gid=25663111

We want to move this to the handbook, and we want the following properties:

1. Renaming a criterion for a level should be easy (not involving copy and paste for every service).
2. Adding a new criterion for a level should be easy (automatically updating all existing evaluations with the new level).
3. Removing a criterion for a level should be easy (failing the build?).
4. Updating level 'nicknames' should be easy.

This will let us iterate on the model itself while avoiding as much busy-work as possible.