Automatically update submodules when other repos push to `main`

Plone Documentation versions affected

• 5.x
• 6.x

Description

Currently whenever there is a push to master for either repo plone/volto or plone/plone.restapi, we manually need to remember to update the submodule. See https://6.docs.plone.org/contributing/index.html#editing-external-package-documentation, Step 7.

cd ../..

# For plone.restapi
git add submodules/plone.restapi
git commit -m "Update submodules/plone.restapi tip"

# For volto
git add submodules/volto
git commit -m "Update submodules/volto tip"

git push

It would be great to handle this automatically in GitHub Actions.