treewide: replace custom mdbook rules with rules_rust extension

Change-Id: I342080f27cf3b41d0522aec7142b35ab53f1e24e
Reviewed-on: https://review.monogon.dev/c/monogon/+/4256
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/handbook/book.toml b/metropolis/handbook/book.toml
new file mode 100644
index 0000000..5a36569
--- /dev/null
+++ b/metropolis/handbook/book.toml
@@ -0,0 +1,5 @@
+[book]
+title = "Metropolis Handbook"
+authors = ["Monogon Project Authors"]
+language = "en"
+multilingual = false