Bootstrap `go-pkg`

This project will also include go SDK higher-level types, like a wasi-sockets socket implementation or http. These will live under `pkg`.

- [x] Update README for `go-pkg`
- [x] Move over vanity, `/docs`
- [x] Move over wit-bindgen source. `/pkg` <- migrate wit-bindgen source
- [ ] Write release process
- [ ] Single tag for wit-bindgen and higher-level Go modules where we have 1 release for all packages in this repo

Then once we have types here, let's migrate wit-bindgen to use these types. We can use a `go.mod` that depends on the types here or use a git submodule over in wit-bindgen for the unit tests.