Add a quickstart tutorial

We should add a quickstart tutorial similar to the [Gin tutorial](https://github.com/golang/tour/blob/master/tutorial/web-service-gin.md) ([rendered page](https://go.dev/doc/tutorial/web-service-gin)) on the [ORAS website](https://oras.land/docs/client_libraries/go), including:
  - How to import `oras-go v2`.
  - How to write basic code using `oras-go v2` APIs.
  - How to run the code.