doc: Expand solver explanations
Context
Introduce elements from Sik Cambon Jensen's Thesis "Design and Implementation of Crossword Compilation Programs" and Harvey's paper "Limited Discrepancy Search". I have not finished reading them, I'll update this MR as I progress. Not blocking for 0.3.
What has changed?
Main Changes
doc
- Expand "How do crossword solvers work?" page
Edited by Antoine Belvire