Fixes documentation generation CI (#360)

Author: BradyBonnette

.github/workflows/docs.yml

@@ -18,7 +18,8 @@ jobs:
         echo "PATH=$HOME/.cargo/bin:$PATH" >> $GITHUB_ENV
     - name: Install mdbook and other preprocessors
       run: |
-        cargo install mdbook mdbook-variables
+        cargo install mdbook --version 0.4.32
+        cargo install mdbook-variables --version 0.2.1
     - name: Deploy GitHub Pages
       run: |
         # This assumes your book is in the root of your repository.

doc/README.md

@@ -2,16 +2,16 @@
 
 The book is built [using `mdbook`](https://rust-lang.github.io/mdBook/index.html).
 
-Install mdbook.
+Install mdbook -- exact version is required since (at the time of this writing) there were some compatibility issues.
 
 ```bash
-cargo install mdbook
+cargo install mdbook --version 0.4.32
 ```
 
-Install mdbook-variables preprocessor
+Install mdbook-variables preprocessor -- exact version is required since (at the time of this writing) there were some compatibility issues.
 
 ```bash
-cargo install mdbook-variables
+cargo install mdbook-variables --version 0.2.1
 ```
 
 Serve the book locally and open your default browser.