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>

.github/workflows/install-mdbook/action.yml

@@ -22,6 +22,10 @@ runs:
       run: cargo install mdbook-i18n-helpers --locked --version 0.2.3
       shell: bash
 
-    - name: Install exerciser
+    - name: Install mdbook-exerciser
       run: cargo install --path mdbook-exerciser --locked
       shell: bash
+
+    - name: Install mdbook-course
+      run: cargo install --path mdbook-course --locked
+      shell: bash