fix: Update broken links in get-the-book.md

Change .md file references to MkDocs clean URLs:
- book-cover.md → book-cover/
- getting-started/quickstart.md → getting-started/quickstart/
- examples/simple-chatbot.md → examples/simple-chatbot/

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: 2 people

docs/get-the-book.md

@@ -70,9 +70,9 @@ We'd really appreciate if you could:
 
 Prefer to read in your browser? The entire book is available online:
 
-- [Start Reading](book-cover.md) - Begin with the cover
-- [Quick Start](getting-started/quickstart.md) - Jump straight to code
-- [Examples](examples/simple-chatbot.md) - See it in action
+- [Start Reading](book-cover/) - Begin with the cover
+- [Quick Start](getting-started/quickstart/) - Jump straight to code
+- [Examples](examples/simple-chatbot/) - See it in action
 
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