Enable dependabot to keep docs dependencies updated

### Checklist

- [x] I added a descriptive title
- [x] I searched open requests and couldn't find a duplicate

### What is the idea?

Turn on dependabot for the requirements.txt file to check that the versions are updated weekly.

### Why is this needed?

So manually bumping requirements isn't needed.

### What should happen?

Add a dependabot `.github` workflow.

### Additional Context

It looks like this was started with https://github.com/conda/conda-docs/pull/991

Start with requirements.txt and the .github configuration, then review pull request #991 because the issue says this work may already have started there. Add the Dependabot configuration so requirements.txt is checked weekly, and verify that the resulting update schedule matches the request.

Written by the indexing model from the issue text.