🤡 Add "make bake" command (#97)

I feel like since this is bread related, release should be renamed to
"bake". So `make bake` will bake you the final book versions.

.github/workflows/validate.yml

@@ -13,7 +13,7 @@ jobs:
             apk add make
             cd book
             make build_pdf
-      - name: Test building release versions
+      - name: Test baking the release versions
         uses: xu-cheng/texlive-action/full@v1
         with:
           run: |
@@ -22,4 +22,4 @@ jobs:
             tar xzf kindlegen_linux_2.6_i386_v2_9.tar.gz
             mv kindlegen /usr/bin
             cd book
-            make release
+            make bake