Added Apollo's Explorer Sandbox interface

[rwillians]

Playground project has been deprecated in Dec 2022 giving space to Apollo's Explorer embedded Sandbox (free -- no account required).

In this PR:

• Added support for Apollo's Explorer Sandbox interface (:apollo_explorer_sandbox) and updated docs;
• Minor refac on Absinthe.Plug.GraphiQL's private functions;
• Bumped absinthe_plug version from 1.5.8 to 1.6.0.

[rwillians]

Apollo Explorer Sandbox isn't working locally for subscriptions anymore -- using websockets in general.

The root cause seems to be associated to the fact that the Origin in the upgrade request is an Apollo hostname and, since the local environment isn't server using SSL (who uses SSL locally??), the browser (chromium-based in my case) aborts the request before it even hits the server.

No to mention the Sandbox tracks a lot of data, which isn't nice.

For those reasons, I'm closing my PR.
For whoever interests in using Apollo Sandbox despite of those reason, you can use it from https://studio.apollographql.com/sandbox/explorer/