Add instructions on how to import public-api Golang client

[easyCZ]

Currently it's not possible to directly use the Public API client defined in https://github.com/gitpod-io/gitpod/tree/main/components/public-api/go

We need to add instructions on how to import the package in go.mod. This will require a re-write rule for now, until we figure out a better package structure.

[easyCZ]

Instructions are here

I've validated the package can now be imported directly with

go get -u github.com/gitpod-io/gitpod/components/public-api/go

This is now complete.