[Feature Request][Go]: Demonstrate building a pipeline from scratch using Beam Go

### What would you like to happen?

We have plenty of examples for the Go SDK, and a starter project, but no "building a pipeline from scratch" guide. Running prebuilt examples aren't as illustrative as a step by step introduction.

The example should remain wordcount, but clearly introducing each part one by one would help users understand their pipelines.

### Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

### Issue Components

- [ ] Component: Python SDK
- [ ] Component: Java SDK
- [X] Component: Go SDK
- [ ] Component: Typescript SDK
- [ ] Component: IO connector
- [X] Component: Beam examples
- [ ] Component: Beam playground
- [ ] Component: Beam katas
- [X] Component: Website
- [ ] Component: Spark Runner
- [ ] Component: Flink Runner
- [ ] Component: Samza Runner
- [ ] Component: Twister2 Runner
- [ ] Component: Hazelcast Jet Runner
- [ ] Component: Google Cloud Dataflow Runner