docs: drop removed multilingual field from book.toml

mdbook 0.5 rejects the field; serve refused to start.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Christina Quast <christina.quast@9elements.com>
Claude-Session: https://claude.ai/code/session_01XTg1L4TfJT9qpYVqZEc9bp

docs/book.toml

diff --git a/docs/book.toml b/docs/book.toml
index a613c4e..0fa6b36 100644
--- a/docs/book.toml
+++ b/docs/book.toml
@@ -4,7 +4,6 @@
 [book]
 authors = ["OpenProt Team"]
 language = "en"
-multilingual = false
 src = "src"
 title = "OpenProt Documentation"