Add mdbook-course to handle parsing frontmatter (#1224)

In v2 of the course, I'd like to include an estimate of the time to be
spent on each segment in the Markdown file. I think a good place for
such metadata is in the frontmatter.

For review purposes, though, I just want to display that information.
So, this is a start at a new mdbook preprocessor that just separates out
the frontmatter and includes it in a `<pre>` block. Eventually, I'd like
to parse it and put the time in the speaker notes.

---------

Co-authored-by: Martin Geisler <martin@geisler.net>

mdbook-course/src/lib.rs

@@ -0,0 +1,15 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+pub mod frontmatter;