Add write permissions for the deployment workflow

This seems related.
https://github.blog/changelog/2023-02-02-github-actions-updating-the-default-github_token-permissions-to-read-only/

Author: daaa1k

.github/workflows/mdbook.yml

@@ -5,6 +5,9 @@ on:
     branches: [master]
   pull_request:
 
+permissions:
+  contents: write
+
 jobs:
   deploy:
     runs-on: ubuntu-latest