slides: add pages for ch4.2

docs(chapter4.2): add an illustration that shows the effect of CF-Connecting-IP

docs(chapter4.2): remove the work powerful

docs(chapter4.2): simplify the last sentence in verify the fix

docs(chapter4.2): remove unnecessary double quotes

docs(chapter4.2): remove gray cloud because it does not exist

docs(chapter4.2): add missing illustration

docs(chapter4.2): remove soft 404 explanation from ch4.2

docs(chapter4.2): add next steps for securing server and introduction to vulnerability scanning

docs(chapter4.2): add pros and cons section for CDN implementation in global delivery and security

docs(chapter4.2): add security measures to block soft 404 vulnerabilities and malicious bot requests in Nginx

docs(chapter4.2): add Nginx caching optimization for Single Page Applications

docs(chapter4.2): add configuration to restore visitor IPs in Nginx when using Cloudflare

docs(chapter4.2): check speed after enabling CDN

docs(chapter4.2): add dns switch verification