Update "guidance and architecture" docs on opentelemetry.io with how to contribute #291
Description
The scaffolding of the new Guidance and Architecture section on opentelemetry.io is underway in open-telemetry/opentelemetry.io#9531, but the new pages do not include information on how to contribute or propose new blueprints/implementations.
Once #236 is complete and issue templates in the SIG End User repo are ready, we'll need to update the official docs.