Use rust-lang/rust linkchecker on CI.

.github/workflows/main.yml

@@ -12,12 +12,12 @@ jobs:
     - name: Install Rust
       run: |
         rustup set profile minimal
-        rustup toolchain install nightly
+        rustup toolchain install nightly -c rust-docs
         rustup default nightly
     - name: Install mdbook
       run: |
         mkdir bin
-        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.3.4/mdbook-v0.3.4-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
+        curl -sSL https://github.com/rust-lang/mdBook/releases/download/v0.3.5/mdbook-v0.3.5-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=bin
         echo "##[add-path]$(pwd)/bin"
     - name: Report versions
       run: |
@@ -26,3 +26,8 @@ jobs:
         mdbook --version
     - name: Run tests
       run: mdbook test
+    - name: Check for broken links
+      run: |
+        curl -sSLo linkcheck.sh \
+          https://raw.githubusercontent.com/rust-lang/rust/master/src/tools/linkchecker/linkcheck.sh
+        sh linkcheck.sh --all nomicon