Document "Support for subdoc array and field operations" for 7.6

PR: https://github.com/couchbaselabs/docs-devex/pull/140

Preview page: https://preview.docs-test.couchbase.com/subdoc-preview/server/current/eventing/eventing-advanced-keyspace-accessors.html#advanced-subdoc-array-op

Document new content for Eventing for the 7.6 Server release.

Support for subdoc array and field operations:

• Supports subdocument capabilities in line with current SDKs
• Lowers network traffic and improves performance
• Array operations have no concurrency issues and can be performed without the need to check CAS

Slide: https://docs.google.com/presentation/d/11FpUOa2iH1yVErPHEbVe2IcU3gs5RTNGqcAP4SrTAUw/edit#slide=id.g26044022064_0_0