Support `HASURA_GRAPHQL_REDIRECT_GET_REQUESTS` environment variable

### Is your proposal related to a problem?

I'm serving a public API with Hasura which I would like to make easy for developers to start using. Right now, developers who are new to the API sometimes attempt to navigate to `https://api.example.com/`, where they see only:

```
{"path":"$","error":"resource does not exist","code":"not-found"}
```

Developers also occasionally send GET requests to `/v1/graphql`, where they see the same error. (Which can be particularly confusing – its plausible that they made a valid request but the server didn't find any results.)

### Describe the solution you'd like

I'd like Hasura to send back a 302 redirect to a helpful URL from the `HASURA_GRAPHQL_REDIRECT_GET_REQUESTS` environment variable, e.g. `https://docs.example.com` or `https://try.example.com` (where I've set up [graphql-playground](https://github.com/graphql/graphql-playground) preset to my API).

If Hasura is considering rendering some sort of standard, more descriptive error message on GET requests to `/v1/graphql`, it might instead be a good idea to have two environment variables, e.g. `HASURA_GRAPHQL_REDIRECT_GET_ROOT` (for redirecting traffic from `/`) and `HASURA_GRAPHQL_REDIRECT_GET_ENDPOINT` for redirecting GET traffic from `/v1/graphql` and other endpoints (`/v1beta1/relay`, etc.). A descriptive error message would be sufficient for GET requests to `/v1/graphql`, but I would still like to redirect users from the root `https://api.example.com/` to my `https://try.example.com/`.

### Describe alternatives you've considered

If I was running my own infrastructure, I could do this myself, but since I'm using Hasura Cloud, I can't even e.g. configure Kubernetes ingress rules or CDN rules. I'm stuck with the error messages unless I migrate off of Hasura Cloud.

### If the feature is approved, would you be willing to submit a PR?

Certainly! (Though I'm very new to Haskell, so I'd appreciate hints about how best to implement it.)

Start by tracing request handling for GET / and /v1/graphql, the entry points named in the issue; no implementation files or tests are identified in the payload. Done means that the requested environment variable can configure a 302 redirect to its URL for the intended GET requests, with the behavior verified for the relevant endpoints.

Written by the indexing model from the issue text.