docs/howto: Use a custom registry with the Go API

[jpluscplusm]

We don't currently cover the use of a custom module registry with the Go API either on https://cuelang.org/docs/tutorial/using-modules-with-go-api/ or https://cuelang.org/docs/tutorial/working-with-a-custom-module-registry/.

We should address this -- possibly by including an example on the latter page, although that page is already quite long.

Original Discord thread that contains some useful pointers:

https://discord.com/channels/1238032673109311508/1324707977126608969/1324707977126608969

go api with custom registry
[11:56] 
OP
 Viacheslav Terdunov: There's example of using Go API with the Central modules registry https://cuelang.org/docs/tutorial/using-modules-with-go-api/#step-7

Could you please help me understand how to work with a custom registry?
I'm not sure if this is the correct way to build a modregistry.Registry instance
How can I configure the registry address and pathEncoding: "hashAsTag" (I'm using AWS ECR as an OCI repo)

I've tried something like this
resolver, _ := modconfig.NewResolver(nil)
client := modregistry.NewClientWithResolver(resolver)
reg, _ := modcache.New(client, "")

# then load
buildInstances := load.Instances(cuesFiles, &load.Config{
    Dir:      ".",
    Registry: reg,
})
 
[12:54]mvdan: have you seen https://pkg.go.dev/cuelang.org/go/cue/load#example-package-ExternalModules ?
[12:56]mvdan: cuelang.org should have a little bit of content about using the Go API with custom registries. https://cuelang.org/docs/tutorial/using-modules-with-go-api/ covers the Go API, and https://cuelang.org/docs/tutorial/working-with-a-custom-module-registry/ covers custom registries, but we don't have a combination of both. I think a small example in the latter page makes sense.
[13:57]mvdan: I also think we should nudge cue/load users towards setting CUE_REGISTRY in load.Config.Env; even though it's a stringy API, it's much higher level and easier to use than chaining modconfig, modcache, and modregistry