Document how to use Dependabot to keep a test suite submodule up to date

(Via @Relequestual here)

Implementers may appreciate a GitHub action which automatically sends PRs to their repo to update the test suite.

As a concrete example, python-jsonschema has a git-subtree of this suite here, and would benefit from automated pull requests whenever the main branch of this repository is updated.

To do this in general, such a github action would need to be configured with the path within a repo where the subtree lives, likely need to support both git-subtree as well as git-submodule (and possibly git-subrepo), and allow for passing options to the subtree/submodule command (e.g. if someone wanted a git subtree pull --squash to be the PR).

It's unclear that this should live in this precise repo rather than an adjacent one, but putting it here to track/ack the request.

Review the referenced python-jsonschema git-subtree example and the request's Dependabot context first. Clarify whether the guidance belongs in this repository or an adjacent one, then define the expected documentation for git-subtree and git-submodule updates, repository-path configuration, and command options; completion requires an agreed scope and usable instructions.