Dokusan

- Python 3.11 compatibility (by ppdms)
- Include Python 3.11 in GitHub Actions

- Rename `entities` -> `boards`
- Fix StopIteration sporadically raised by Unique Rectangle
- Rewrite renderers
- Bump coverage to 100%

Search for Locked Candidates in all groups

- Rename `min_rank` to `avg_rank`
- Split CI workflows
- Add badges to README

- Sudoku can be represented as string
- Sudoku can be initialised from string
- New step-by-step solver
- Fix naming:
* `techniques.Result` -> `techniques.Step`
* `generator.generate` -> `generators.random_sudoku`
- `BoxSize` is required in every Sudoku constructor

- Sudoku Generator
- Sudoku Ranking
- Backtracking based solver
- Elimination based solver