Add Deep Generative Models module

[MHHamdan]

Adds a self-contained Deep-Generative-Models knowledge module to the repository.

Includes:

• a landing README with the family map and the generative trilemma
• seven concept notes: foundations; autoregressive models; normalizing flows; variational autoencoders; GANs; energy-based models; diffusion and score-based models
• a math page covering MLE↔KL, change of variables, the ELBO, and the score
• a Mermaid taxonomy diagram
• a glossary and canonical references list

All content is original and cites the primary papers; no source text is reproduced.