Enable renovate in the sphinx starter pack?

I think it would make sens to enable renovate in this repo.

Over time, teams must re-sync their repo with this starter pack.

Since this repo doesn't use renovate, we sometimes have to downgrade Ubuntu image or action versions.

As an example, the [markdown-style-checks.yml](https://github.com/canonical/sphinx-docs-starter-pack/blob/main/.github/workflows/markdown-style-checks.yml) is using Ubuntu 22.04 and actions/checkout@5.

Enabling renovate would create PR automatically to bump versions that could be bumped. And keep this repo with the latest actions and Ubuntu images.