docs: rewrite CNN architecture chapter in the new teaching format

docs: finalize recurrent-networks docs

docs: soften recurrent-networks migration wording

nav: add minimal migration hints for recurrent-networks

nav: refine 2013 LSTM timeline wording

nav: link timeline RNN entries to recurrent-networks

docs: tighten recurrent-networks EN wording

nav: add recurrent-networks to Phase 02 English listing

nav: add recurrent-networks to Phase 02 listing

content: add recurrent-networks module (English)

content: refine recurrent-networks Mermaid labels

content: refine recurrent-networks README

plan: define cnn chapter rewrite

docs: add backpropagation entry to Prerequisites index

docs: add cross-link from deep-learning-basics to backpropagation module