[Task] Documentation for GraphQL

[andrewazores]

GraphQL for 2.1.0 is feature-complete after #872 is merged. We need upstream development-level docs for this feature, including an entry in HTTP_API.md.

cryostatio/cryostat#872 (comment)

It probably makes more sense to document the various queries/mutations implemented and the possible filter inputs and the behaviour of using multiple filter input fields somewhere separately from HTTP_API.md, since that's a lot of content specific to just one high-powered API endpoint. The existing GRAPHQL.md could be a good candidate for reuse here.

[andrewazores]

My current thought is that we should simply document the top-level GraphQL endpoint(s), ex. POST /api/v2.2/graphql as the entrypoint, and have a high-level description of what this API surface is used for with a few useful examples. I don't think we should try to fully, exhaustively document this API surface in a markdown document however. Rather, we should simply refer the reader to the queries.graphqls and types.graphqls files. These already describe the API in good detail to readers who have familiarity with GraphQL.