docs: Documentation of the pixel-wise multi-label evaluation of the document layout analysis

[nikos-livathinos]

This is a documentation of the pixel-wise multi-label evaluation method we use for the document layout analysis.
It covers the following topics:

• Objectives.
• Confusion matrix: Structure, build algorithm, semantic.
• Derivatives: Recall, precision, F1 matrices and vectors.
• Implementation details: Binary representation, performance optimizations.
• Examples

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