Move patterns to new maturity levels

[spier]

To start using the new pattern maturity levels outlined as part of the new contribution process, we want to move all existing patterns to a folder structure, indicating which maturity each pattern is in.

As a first step we will:

• Move all Donut patterns to folder 1-initial
• Move all other patterns to folder 2-structured
• Fix all links in the main README to point to the new pattern locations
• Adapt the grouping in the main README to fit to the maturity levels (we have 3 maturity levels but currently 5 groups)

This is a prerequisite for then confirming which patterns have enough known instances (companies that are using this pattern), and can hence be moved from 2-structured to 3-validated. (see #186)

Side Note

As we have a couple of open PRs for existing patterns, moving the patterns around will lead to merge conflicts at some point in the future.

We will try to reduce the extra work for the authors of these PRs as much as we can. We have to figure out how exactly to do that when we get there.

[spier]

I got a WIP PR open for this. Feel free to assign me to this issue if you like.

[spier]

This work has been completed with PR #188.