Add docs for adding renovate and remove noise from dependabot

Steps it should cover:
- Installing the bot on your repo
- Configurating GitHub for vulnerability alerts: https://docs.renovatebot.com/configuration-options/#vulnerabilityalerts

Also need to put a minimal `.github/renovate.json` in, and strip down `dependabot.yml` to only include critical vulnerabilities, and a test that makes sure any action we put in the template is ignored downstream