Enable single-commit option for doc deployment (#5138)

Closes #5092.

The `force` option wasn't needed as it is [already enabled by default](https://github.com/JamesIves/github-pages-deploy-action#optional-choices) (however I can add it if it would be better to have it explicitly specified).

Author: Kanabenki

.github/workflows/docs.yml

@@ -54,3 +54,5 @@ jobs:
         with:
           branch: gh-pages
           folder: target/doc
+          single-commit: true
+          force: true