Documentation for Concent Settlement Algorithm

[cameel]

Resolves #996.

This is the final version of the implementation docs for the settlement algorithm. The content is identical to #996 and to the draft at Google Docs: https://docs.google.com/document/d/12eisUDBGUuAvlGErVaUXtRmBybIB89ZLdlP5IgDn43o. There might be slight formatting differences due to different formats (document vs markdown vs reStructuredText). When this pull request is merged the blueprint and the draft will be made read-only/closed and will no longer be updated.

To build and view the rendered version:

cd docs
pip install -r requirements.txt
make html
xdg-open build/html/index.html

Since everybody has already read the draft, this pull request should not require an in-depth review but feel free to report any formatting errors or discrepancies between the docs and the code that have not been spotted earlier.

NOTE: The main file is docs/source/settlement-algorithm.rst and that's the one you should focus on. There rest are diagrams. The XML files can be opened with draw.io. SVG files are a direct export from the corresponding XML files. I'd prefer not to have them in the repo but unfortunately support for automatic export is severely lacking in diagramming tools.

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