Add dependabot

[diegoferigo]

Frankly speaking, dependabot has always annoyed me mainly because it used to open too frequent PRs. However, I've recently discovered that the rate of updates can be configured granularly, and it's also possible to merge together all necessary updates in a single PR.

I'd like to give it a try by setting one update per month to the actions part of our GitHub workflows. We can fine tune it in the future, if needed.

• https://docs.github.com/en/code-security/dependabot
• github-actions: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#github-actions
• groups: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups

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📚 Documentation preview 📚: https://jaxsim--266.org.readthedocs.build//266/

[flferretti]

LGTM, thanks! Please check the comment I left as it might be crucial for the intended behavior