Upgrade gb-asm-tutorial to use mdbook 0.5

This commit updates the various unpublished crates in the repo to be set
up according to the new requirements imposed by mdbook 0.5. In addition,
it updates the build and deploy workflow to account for these changes as
well.

As part of this update, the theme/index.hbs had to be updated to work
with some syntax changes made for mdbook 0.5. The style looked off in my
browser, so I would appreciate some double checking on this front.

Author: mtthgn

.github/workflows/build-and-deploy.yml

@@ -9,32 +9,27 @@ permissions:
   id-token: write
 
 env:
-  MDBOOK_LINKCHECK_VER: 0.7.7
+  MDBOOK_VERSION: 0.5.2
+  MDBOOK_LINKCHECK2_VERSION: 0.12.0
 
 jobs:
   build:
     runs-on: ubuntu-latest
     steps:
+      - uses: dtolnay/rust-toolchain@stable
       - name: Checkout gb-asm-tutorial
         uses: actions/checkout@v2
         with:
           path: gb-asm-tutorial
 
-      - name: Install mdbook
-        uses: peaceiris/actions-mdbook@v1
-        with:
-          mdbook-version: 0.4.52
+      - name: Install mdbook dependencies
+        run: |
+          cargo install mdbook@${MDBOOK_VERSION}
+          cargo install mdbook-linkcheck2@${MDBOOK_LINKCHECK2_VERSION}
 
       - name: Install static-sitemap-cli
         run: npm install static-sitemap-cli
 
-      # FIXME: Keep this up to date
-      - name: Install mdbook-linkcheck
-        run: | # `-L` because GitHub performs a redirection
-          curl -L -o mdbook-linkcheck.zip "https://github.com/Michael-F-Bryan/mdbook-linkcheck/releases/download/v$MDBOOK_LINKCHECK_VER/mdbook-linkcheck.x86_64-unknown-linux-gnu.zip"
-          unzip mdbook-linkcheck.zip mdbook-linkcheck
-          chmod +x mdbook-linkcheck
-
       - name: Cache build dir
         uses: actions/cache@v4
         with:
@@ -47,8 +42,7 @@ jobs:
         working-directory: gb-asm-tutorial/
         env:
           MDBOOK_BUILD__CREATE_MISSING: "false" # Prevent creating missing files in SUMMARY.md
-          MDBOOK_OUTPUT__LINKCHECK__COMMAND: "../mdbook-linkcheck"
-          MDBOOK_OUTPUT__LINKCHECK__OPTIONAL: "false"
+          MDBOOK_OUTPUT__LINKCHECK2__OPTIONAL: "false"
         run: |
           mdbook build
           for f in po/*.po; do