Set working hostnames for ENSApi docs

[lightwalker-eth]

Background

Goals

1. By default, all the auto-generated "curl" examples should just work. Likewise, all the interactive "Try it! / Send" buttons should just work.
2. By default, all queries should go to https://api.alpha.ensnode.io
3. Nice to have: Can we have a "hostname picker" of some kind, where you could interactively pick between the following options:
   1. https://api.alpha.ensnode.io
   2. https://api.alpha-sepolia.ensnode.io

[notrab]

@lightwalker-eth

1. Yeah, we need to make sure the service that is exposing the OpenAPI also exposes the URL in which it is present on. If you see what we have in production, https://api.alpha.green.ensnode.io/openapi.json, you'll see the URL is localhost here. In my opinion, this should be https://api.alpha.green.ensnode.io since that's the API it is deployed to, but it then means we have duplicates in (2);
2. Yes, agreed. If we always point the docs to the released production API spec, then yeah this would be fine.
3. 3. I've just tried to see if this is possible with Mintlify, and it is. Below are some screenshots of how it looks when you add multiple servers to the spec: