docs: complete Ch 9 exercises

docs: refine Ch 9 notebook

docs: update chapters index page

docs: finish Ch10: theory and exercises

docs: complete Ch10 theory

docs: start Ch 10 theory

docs: initialize Ch 10 for study

docs: expand Sakila appendix with DB diagrams

docs: expand Ch 6 with comprehensive text

docs: complete Ch 9 theory

docs: continue Ch 9 theory

docs: start Ch 9 theory

docs: update SQL index with new functions

docs: complete Ch 8 exercises

docs: complete Ch 8 theory

docs: rewrite Polars setup guide section

docs: complete Simulative Lesson 8

docs: finish Simulative Lesson 7 practice

docs: start Simulative Lesson 7 practice

docs: finish Simulative Lesson 7 theory

docs: fix security in Ch 8 and update Simulative section

docs: expand Appendix, index and polish about pages

chore: update book layout and configuration