Add Renovate support

[mthalman]

• Adds Renovate pipeline infrastructure to eng/common for automated dependency updates
  • Creates reusable job template for running Renovate against GitHub repositories
• Adds an Arcade-specific Renovate pipeline that updates the version of Renovate being referenced in the job template
• Updates the Renovate documentation:
  • more thorough
  • onboarding instructions
  • references to the pipeline-related files being added in this PR
  • common patterns

[mmitche]

Approved with minor comment.

[lbussell]

[lbussell]

Question: did you consider creating an "extends" template? Then maybe the minimal template for adopting renovate could be even smaller.

Hypothetical example

   trigger: none

   schedules:
   - cron: '0 8 * * 1'
     displayName: Weekly Renovate Run
     branches:
       include:
       - main
     always: true

   extends:
     template: /eng/common/idk-wherever/renovate.yml@self
     parameters:
       renovateConfigPath: my-renovate-config.json
       gitHubRepo: dotnet/arcade
       dryRun: false
       forceRecreatePR: false

Maybe something to consider for a follow-up

[mthalman]

Good suggestion. Updated: f0af622

[mthalman]

I'm going to revise this some more to remove the dependency on ghcr.io since that will be impacted by network isolation eventually.