Set up dependabot auto-merge policies

[gewarren]

Describe the issue or suggestion

Enable automatic merge of dependabot PRs via workflow in these repos:

• dotnet/docs
• dotnet/dotnet-api-docs
• dotnet/aspnetcore.docs
• dotnet/docs-maui
• dotnet/docs-mobile
• dotnet/docs-tools
• dotnet/docs-actions
• dotnet/docs-desktop Doesn't have dependabot

---

Associated WorkItem - 636396

[gewarren]

Most repos already have a workflow for this. Ensure that the dependabot/fetch-metadata action is allowed in each repo.Dependabot metadata step isn't required if you don't need the output.