[Stage] Cleanup Discoverable related documentation

[Skillz4Killz]

Hello 👋

Since stage discovery was removed, i believe a lot of documentation is left in limbo which should either be fixed or deprecated.

Both privacy_level and discoverable_disabled seem to be something that should be deprecated.
https://discord.com/developers/docs/resources/stage-instance#stage-instance-object-stage-instance-structure

Guild member update events and i believe voice state updated were changed to make joined_at nullable to make discovery work. This makes for a more annoying api. Since discovery is gone and a stage can only be joined by a member can we document this properly by making it no longer nullable. https://discord.com/developers/docs/topics/gateway#guild-member-update-guild-member-update-event-fields

Thanks 😄