Update Chaincode-as-a-Service docs to reflect the new default ccaas builder

The Fabric docs on [CCaaS external builders](https://hyperledger-fabric.readthedocs.io/en/latest/cc_service.html#modify-the-peer-core-yaml-to-include-the-externalbuilder) are out of date and still too complicated for general consumption. The External Builders section includes enough detail for users that seek to override the embedded defaults with a custom cc compiler.

Update this section of the docs / guide to reflect the new, default external `ccaas` builder embedded into the Fabric > 2.4.1 peer image. This is also a good location in the docs trail to address invocation of a CC running on the local host via reverse proxy in cloud-based environments, or direct connection to a CC endpoint attached to an IDE.

The new default builders make it really, really easy to work with chaincode. The code is wired up, and works great. The docs still present a bit of an uphill climb.