Docs: Update the "third party instrumentation" text to explain that the repo is now archived

[fallwith]

The Third party instrumentation reads as though the extends_newrelic_rpm repo is still actively maintained, but the repo is in fact archived. The docs should be updated to a) better align with the fact that the repo is now archived, and b) better align with the fact that the main repo is now open source (as opposed to when the doc was written) and that all 3rd party enhancements are quite welcome to be proposed as 1st party enhancements via our PR process.

[workato-integration]

https://issues.newrelic.com/browse/NEWRELIC-5077