Import wiki from original Darglint

[akaihola]

Fixes #3 partially.

Bringing in the Wiki documents and images from the Design section of the original Darglint wiki.

To do:

• [ ] move the documents to the docs/ folder
• [ ] build Graphviz images into ~~PNG~~ SVG as part of documentation build
• [x] decide whether to include those ~~PNG~~ SVG images in the repository ~~or to host them from somewhere else~~
• [ ] implement decided storage for the ~~PNG~~ SVG images