Update docs and guidance for contributions

[bamurtaugh]

Update resources and documentation to help with community contributions:

• Add a new PR template (based on the great work from forem)
• Add links to collection-index.yml from the Features and Templates pages (we already link to it from the Collections page)
• Update the Contribution Guides that appear on containers.dev and are part of this backing repo
  • Add info on contributing Templates and Features
  • Make them consistent with each other (in content and formatting; i.e. the one displayed on containers.dev was missing the latest formatting info we added to the other guide)

This is a simpler version way to achieve the goals from #354.