Comment PRs with updated schedule information (#1576)

This adds a GH action to add a comment to every PR giving the updated
course schedule with the PR merged.

To accomplish this, I broke `mdbook-course` into a library and two
binaries, allowing the mdbook content to be loaded dynamically outside
of an `mdbook build` invocation.

I think this is a net benefit, but possible improvements include:
* diffing the "before" and "after" schedules and only making the comment
when those are not the same (or replacing the comment with "no schedule
changes")
* including per-segment timing behind `<details>` (with a few minutes
effort I couldn't get this to play nicely with the markdown lists)

---------

Co-authored-by: Martin Geisler <mgeisler@google.com>

.github/workflows/course-schedule.yml

@@ -0,0 +1,27 @@
+name: "Course Schedule Updates"
+on:
+  pull_request:
+    paths:
+      - "src/**.md"
+
+jobs:
+  course-schedule:
+    runs-on: ubuntu-latest
+    name: Make Course Schedule Comment
+    permissions:
+      pull-requests: write
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+
+      - name: Setup Rust cache
+        uses: ./.github/workflows/setup-rust-cache
+
+      - name: Generate Schedule
+        run: cargo run -p mdbook-course --bin course-schedule > course-schedule.md
+
+      - name: Comment PR
+        uses: thollander/actions-comment-pull-request@v2
+        with:
+          filePath: course-schedule.md
+          comment_tag: course-schedule