Погружение в паттерны проектирования книга

(cherry picked from commit e762fa6c90fec868c8698b22f75eadf713d9b5bc)

#18. Added deadline links (v2.5)

Semester 2 index.mdx upd & shortcuts update

Merge remote-tracking branch 'origin/master'

Merge pull request #10 from nawinds/navigation

Navigation, dark theme, deadlines loading fix & snowflakes background

Update index.mdx