Add dependabot to changelog

docs/devel/changelog_staging.md

@@ -5,3 +5,4 @@
 
 # Changelog for next release
 - Added ability to define `$CTYPESGEN_PIN` when building sdist via `./run craft pypi --sdist`, which allows to reproduce our sdists when set to the head commit hash of `pypdfium2-team/ctypesgen` at the time of the build to reproduce. Alternatively, you may patch the relevant `pyproject.toml` entry yourself and use `PDFIUM_PLATFORM=sdist python -m build --sdist` as usual.
+- Set up Dependabot for GH Actions. Updated dependencies accordingly.