Add Dependabot configuration for automated dependency updates

[rscohn2]

Add Dependabot configuration for automated dependency updates

Summary

• Adds .github/dependabot.yml to automate dependency updates
• Configures weekly checks for Python dependencies (docs/requirements.txt)
• Configures weekly checks for GitHub Actions across all workflows
• Dependabot will maintain SHA pinning format for GitHub Actions
• Replaces default dependabot, that was updating requirements.doc, but was not updating github actions. e.g. Update GitHub Actions to latest versions in documentation workflow #729

Benefits

• Automated PRs for security patches and bug fixes
• Continues current SHA pinning best practice for Actions
• Reduces manual maintenance burden
• Ensures dependencies stay current

🤖 Generated with Claude Code

[vmalia]

@rscohn2 We already get automated updates from dependabot as seen in the recent PRs like #720 #721 #724 and #725. Can you please share the motivation for this PR and how it will affect the existing mechanism?

[rscohn2]

Can you please share the motivation for this PR and how it will affect the existing mechanism?

I updated the description to answer your question:
Replaces default dependabot, that was updating requirements.doc, but was not updating github actions. e.g. #729

You need a dependabot.yml to handle github actions, and if you have a yml file, you have to declare everything, including the requirements.txt, which was being done by default before.