feat(ci): add scheduled job to update all themes

[emilioziniades]

I recently merged a PR to the zola-bearblog theme, and noticed that the readme hadn't updated on the zola website. After some investigation I discovered this repo. Instead of making the PR here to update all submodules, I thought it would make sense to create a github action to do the same thing.

It runs daily at 5:51 UTC. The github docs recommended not scheduling actions on the hour, so in search of a random time during the day, I picked the time of the first commit to zola.

The action commits the results to master if there are any changes, and the author of the commit is "[email protected]", see example here.

It can also be triggered manually from the github actions UI.

[Keats]

Looks good can we put it in a weekly schedule? It will automatically create a PR on the zola repo and I don't want once a day.

[emilioziniades]

Sure, I've updated the schedule to run every Monday at 5:51 UTC

[Keats]

thanks!