Add docs for adding renovate and remove noise from dependabot #299

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