[mdBook template]: Fix issue causing mdBook pipeline to return failure when pushing changes that don't modify the mdBook (#88)

`git commit` returns 1 if there is nothing to commit, meaning that if you `git push` any changes that don't modify the mdBook, `git` will return failure because there is nothing to commit, causing the pipeline to return failure.
Adding a basic `git diff` check resolves this issue.

Reviewed-on: https://codeberg.org/Codeberg-CI/examples/pulls/88
Reviewed-by: crapStone <codeberg@crapstone.dev>
Co-authored-by: carnivorecookies <carnivorecookies@noreply.codeberg.org>
Co-committed-by: carnivorecookies <carnivorecookies@noreply.codeberg.org>

Mdbook/.woodpecker.yaml

@@ -49,5 +49,4 @@ steps:
       - cp .domains $CI_REPO_NAME || true
       - cd $CI_REPO_NAME
       - git add .
-      - git commit -m "Woodpecker CI ${CI_BUILD_CREATED}"
-      - git push
+      - if ! git diff --cached --quiet; then git commit -m "Woodpecker CI ${CI_BUILD_CREATED}" && git push; fi