metropolis/handbook: init

This is the beginning of the Metropolis Handbook, the main end-user
documentation of Metropolis.

It is built using mdbook, and currently contains only the default
content from `mdbook init`.

Future work: start writing the handbook, build in CI, publish in CI.

Change-Id: I81753350215b2f7aabc17925eadfd20706e1fdb5
Reviewed-on: https://review.monogon.dev/c/monogon/+/202
Reviewed-by: Leopold Schabel <leo@nexantic.com>
diff --git a/metropolis/handbook/BUILD.bazel b/metropolis/handbook/BUILD.bazel
new file mode 100644
index 0000000..72c3490
--- /dev/null
+++ b/metropolis/handbook/BUILD.bazel
@@ -0,0 +1,7 @@
+load("//metropolis/handbook:defs.bzl", "mdbook_html")
+
+mdbook_html(
+    name = "handbook",
+    srcs = glob(["src/**/*.md"]),
+    title = "Metropolis Handbook",
+)