[rust, bazel] separate mdbook into its own crate index

mdbook is only used as binary and we don't use it as a library, so making
it a separate crate index. This allows us to avoid mixing its dependencies
with other OT dependencies.

Signed-off-by: Gary Guo <gary.guo@lowrisc.org>

Author: nbdd0121

third_party/mdbook/BUILD

@@ -0,0 +1,5 @@
+# Copyright lowRISC contributors (OpenTitan project).
+# Licensed under the Apache License, Version 2.0, see LICENSE for details.
+# SPDX-License-Identifier: Apache-2.0
+
+package(default_visibility = ["//visibility:public"])