nginx books: fixed urls, added new books.

[y82]

This PR:

• fixes some book urls on https://nginx.org/en/books.html (not all - some books aren't available on the publisher's website)
• adds 3 more books
• images are a separate commit for easier review

Checklist

Before creating a PR, run through this checklist and mark each as complete:

• I have read the contributing guidelines.
• I have signed the F5 Contributor License Agreement (CLA).
• If applicable, I have added tests that prove my fix is effective or that my feature works.
• If applicable, I have checked that any relevant tests pass after adding my changes.
• I have updated any relevant documentation (README.md and/or CHANGELOG.md).

[pluknet]

As noted in PR's intro text, I'd also trim unavailable books.
That is, after link fixes applied, including previous book editions:

• Nginx HTTP Server
• Nginx 1 Web Server Implementation Cookbook
• Nginx HTTP Server - Second Edition
• Instant Nginx Starter
• Nginx Module Extension
• Nginx High Performance
• Nginx HTTP Server - Third Edition
• Nginx richtig konfigurieren
• NGINX Cookbook - 2019 Edition
• NGINX Cookbook, 2nd Edition

[pluknet]

I prefer to list only the latest edition of every book, similar to c88e9bf.
There's little point to list previous editions.
This would make adding or replacing with the next books:

• NGINX Cookbook, 3rd Edition
• NGINX HTTP Server - Fifth Edition

[pluknet]

The correct publishers link appears to be:
https://www.oreilly.com/library/view/nginx-cookbook-3rd/9781098158422/

(f5 blog post contains some unrelated advertisement and doesn't really provide publishers link)

[thatrobotdev]

Yes, it seems you are right @pluknet. The blog post provides https://www.f5.com/go/ebook/complete-nginx-cookbook as a download link, but it no longer works.