docs: Force light mode syntax highlighting (#11377)

Release Notes:

- N/A

Co-authored-by: Marshall <marshall@zed.dev>

docs/book.toml

@@ -8,6 +8,7 @@ site-url = "/docs2/"
 
 [output.html]
 no-section-label = true
+preferred-dark-theme = "light"
 
 [output.html.print]
 enable = false