Contributing guide needs several updates

[cmaf]

The Contributing Guide needs an overhaul. I am creating a list of suggestions here for ways to improve it:

• Review of contributor roles
• In Github labels and keywords that block PRs, should DNM be added to the table under PR description contains?
• Use Static Checks for Validation has a link to a script from a 1.x-eol branch seems to have been updated, but consider updating image example of failed CI test
• Configure your environment should include info about rust now
• In either this guide or a new getting started guide (see Create a Getting Started Guide for developers #260 ), consider adding a link for advice on how to resolve merge conflicts when rebasing (example)
• The section on porting is huge, we could consider a separate document
• In continuous integration, it would be helpful to have a link listing /test-* shortcuts for maintainers