doc: add async_hooks migration note

Implements https://github.com/nodejs/diagnostics/issues/194#issuecomment-1304043167, a docs addition informing users that the public async_hooks API will be going away at some unspecified future date; and some suggestions for other APIs to use instead. cc @Qard @mcollina @nodejs/diagnostics @nodejs/tsc

This doesn’t commit us to either a timeline or a process for async_hooks going internal, but I think merging in this PR can mark that we’ve decided on the goal we want to reach. Then we can discuss how we want to get there.