Add docs.wasmtime.dev as a CNAME for the Wasmtime docs (#2317)

* Update version of mdbook used in CI

* Configure cname for wasmtime docs

.github/workflows/main.yml

@@ -33,7 +33,7 @@ jobs:
         submodules: true
     - run: |
         set -e
-        curl -L https://github.com/rust-lang-nursery/mdBook/releases/download/v0.3.1/mdbook-v0.3.1-x86_64-unknown-linux-gnu.tar.gz | tar xzf -
+        curl -L https://github.com/rust-lang-nursery/mdBook/releases/download/v0.4.4/mdbook-v0.4.4-x86_64-unknown-linux-gnu.tar.gz | tar xzf -
         echo `pwd` >> $GITHUB_PATH
     - run: (cd docs && mdbook build)
     - run: cargo build -p wasmtime

docs/book.toml

@@ -3,3 +3,6 @@ authors = ["The Wasmtime Project Developers"]
 multilingual = false
 src = "."
 title = "Wasmtime"
+
+[output.html]
+cname = "docs.wasmtime.dev"