Add dependabot config

[LingMan]

Dependabot will check for updatable dependencies and create PRs to update the relevant files.

The cargo section will monitor Cargo.{toml,lock} while the github-actions section monitors any workflow files in .github/workflows. Both sections are set to update on a weekly basis.

@oli-obk This is in response to your comment here about checking for dependency updates in a cronjob.

[oli-obk]

Cool! thanks. Let's try it out and we can adjust settings further depending on how it actually goes