[Task]: Add Go page and example(s) to the IO docs

[damccorm]

What needs to happen?

https://beam.apache.org/documentation/io/developing-io-overview/ has examples and a dedicated page for Java and Python - we should add an example or two for Go. Probably textio and maybe our example streaming pubsub (https://github.com/apache/beam/blob/master/sdks/go/examples/native_wordcap/nativepubsubio/native.go)? We should also add a page like https://beam.apache.org/documentation/io/developing-io-java/ for Go

Issue Priority

Priority: 2

Issue Component

Component: sdk-go