Improve vttablet modes documentation

[deepthi]

We have a user guide for 'Unmanaged Tablet' which tells users how to run vttablet in unmanaged mode. However, the exact functional differences between the various modes are unclear. We have an old doc at https://github.com/vitessio/website/blob/prod/wip-docs/user-guides/vttablet-modes.md which we can resurrect and add more detail to. For instance, we should address how vreplication, online DDL etc behave in unmanaged mode.