Add: dependabot checks for workflow versions

[eddiebergman]

This PR adds dependabot to check for updates to workflow versions, creating PR's to the repo if an update exists. This is checked daily and the documentation for such is included in the file.

We did so in autosklearn and here you can see sample PRs from dependabot

CC @mfeurer

[mfeurer]

This implements a solution for #1153

[eddiebergman]

This implements a solution for #1153

Not exactly, there's a seperate PR for that at #1154. This will bump the workflow action numbers, i.e. the v2 in actions/setup-python@v2