Enable dependabot updates for theme

[max06]

This workflow will semi-automatically update just the docs in the gemfile and the corresponding lock-file.

This doesn't only keep the template updated, it will also create PRs for every repository derived from this template.

Example PR: max06#2

A change to the README.md will come soon, this feature needs a settings change in the created repository.

@just-the-docs/maintainers I'd love to hear your opinions!

[mattxwang]

Thanks for this PR, it's certainly an interesting idea!

Two questions:

• I'm a bit wary of adding more third-party actions, which themselves need to be kept in sync. Could we not instead have a dependabot template action? From my experience, the Ruby support works relatively well, and it's first-party / a part of GitHub
• any idea why the Netlify deploy is failing here? we haven't changed any of the actual build!

[max06]

I have no clue about netlify. Some builds are going through, some are failing.

I'll get deeper into dependabot. Most of the stuff I used here was copied from another project of mine, didn't think about that.

[max06]

Changed to dependabot - I'll wait for the first PR before marking this as ready

[mattxwang]

Just getting to this now -- will this run if it's not merged into main? I don't think so, right?

[max06]

I have merged it into my own main branch of a fork - it should tell us if it works before we ship untested things to users.

[max06]

After fixing a wrong schema, it works now.