Feature: Use dependabot for GHA updates

[erikmd]

Related issue(s) or PR(s):

• Bump coq-community/docker-coq-action from 1.2.2 to 1.2.4 coq-community/run-coq-bug-minimizer#12 (example)

Related project scope(s): GHA

The problem

It would be nice to automatically suggest version changes of GH actions in learn-ocaml's workflows,
so that we can quickly be aware of a new major version of release-please-action or so, and with which features.

Wanted solution

• Config example: https://github.com/coq-community/run-coq-bug-minimizer/blob/master/.github/dependabot.yml

Considered alternatives

N/A

Additional context

N/A