document which graphql APIs are considered stable

[sryza]

Currently, the GraphQL APIs covered by the Python GraphQL client are considered stable. But we don't document this. We can/should document it in these places:

• https://docs.dagster.io/getting-started/releases#releases-and-compatibility
• https://docs.dagster.io/concepts/dagit/graphql#graphql-api

Relevant internal discussion: https://elementl-workspace.slack.com/archives/C03D094F23G/p1689274797390469?thread_ts=1689005416.349499&cid=C03D094F23G