documentation for workload identity with Vault and federation

### Proposal
The documentation for workload identity needs a lot of work; at the moment a bunch of key items (at least, that I consider key items) are missing which makes it very hard to determine what, if any, impact a future upgrade will have. Most notably lacking is an explanation how to set up workload identity across multiple federated clusters. Given the fact that the Vault JWT auth endpoint requires you point it back at the JWKS URL in Nomad, it seems to imply each cluster needs it's own endpoint. This would very much make life incredibly complicated if cluster A goes tits up, and you reschedule everything on cluster B, but now you'll need all the roles required on cluster A defined on the auth endpoint for cluster B as well.

As someone with 4 clusters federated together this is kind of a deal breaker at the moment.

Also lacking is decent pointers and/or info towards how to migrate from the current Vault integration to the new workload identity thing; at least, there is some info available but it's all sort of scattered.

### Use-cases
Making my life easier and letting me decide whether we're going to pin ourselves on Nomad 1.8 or not.

The issue names no files or tests. Start by locating the workload identity, Vault integration, federation, and migration documentation; done means the docs explain setup across federated clusters, JWKS and role considerations, and the migration impact from the existing Vault integration.

Written by the indexing model from the issue text.