third_party/rust: initialize, add mdbook

As we want to use [mdbook](https://github.com/rust-lang/mdBook) to build
documentation, we now have to pull it into the monorepo, alongside
support for Rust in general.

Testing plan: bazel run //third_party/rust:cargo_bin_mdbook. The CI
should also pick this up now.

Change-Id: I6cf5d02d926bb0de61a5c882828accd35f3a1076
Reviewed-on: https://review.monogon.dev/c/monogon/+/201
Reviewed-by: Leopold Schabel <leo@nexantic.com>
diff --git a/third_party/rust/cargo/remote/BUILD.markup5ever_rcdom-0.1.0.bazel b/third_party/rust/cargo/remote/BUILD.markup5ever_rcdom-0.1.0.bazel
new file mode 100644
index 0000000..1ef6a56
--- /dev/null
+++ b/third_party/rust/cargo/remote/BUILD.markup5ever_rcdom-0.1.0.bazel
@@ -0,0 +1,81 @@
+"""
+@generated
+cargo-raze crate build file.
+
+DO NOT EDIT! Replaced on runs of cargo-raze
+"""
+
+# buildifier: disable=load
+load("@bazel_skylib//lib:selects.bzl", "selects")
+
+# buildifier: disable=load
+load(
+    "@rules_rust//rust:rust.bzl",
+    "rust_binary",
+    "rust_library",
+    "rust_test",
+)
+
+package(default_visibility = [
+    # Public for visibility by "@raze__crate__version//" targets.
+    #
+    # Prefer access through "//third_party/rust/cargo", which limits external
+    # visibility to explicit Cargo.toml dependencies.
+    "//visibility:public",
+])
+
+licenses([
+    "notice",  # MIT from expression "MIT OR Apache-2.0"
+])
+
+# Generated Targets
+
+# Unsupported target "hello_xml" with type "example" omitted
+
+# Unsupported target "html2html" with type "example" omitted
+
+# Unsupported target "print-rcdom" with type "example" omitted
+
+# Unsupported target "xml_tree_printer" with type "example" omitted
+
+rust_library(
+    name = "markup5ever_rcdom",
+    srcs = glob(["**/*.rs"]),
+    crate_features = [
+    ],
+    crate_root = "lib.rs",
+    crate_type = "lib",
+    data = [],
+    edition = "2018",
+    rustc_flags = [
+        "--cap-lints=allow",
+    ],
+    tags = [
+        "cargo-raze",
+        "manual",
+    ],
+    version = "0.1.0",
+    # buildifier: leave-alone
+    deps = [
+        "@raze__html5ever__0_25_1//:html5ever",
+        "@raze__markup5ever__0_10_1//:markup5ever",
+        "@raze__tendril__0_4_2//:tendril",
+        "@raze__xml5ever__0_16_1//:xml5ever",
+    ],
+)
+
+# Unsupported target "html-driver" with type "test" omitted
+
+# Unsupported target "html-serializer" with type "test" omitted
+
+# Unsupported target "html-tokenizer" with type "test" omitted
+
+# Unsupported target "html-tree-builder" with type "test" omitted
+
+# Unsupported target "html-tree-sink" with type "test" omitted
+
+# Unsupported target "xml-driver" with type "test" omitted
+
+# Unsupported target "xml-tokenizer" with type "test" omitted
+
+# Unsupported target "xml-tree-builder" with type "test" omitted