validate volume_mounts don't override templates and payloads

In https://github.com/hashicorp/nomad/issues/7796 we determined that `volume_mounts` (both host volumes and CSI volumes) "shadow" the destinations of template, artifact, and dispatch_payload because the mounts are created by the task isolation when we launch the task (ex. by Docker's `mount` option), which happens well after template rendering and other pre-run hooks.

We've documented this behavior at https://www.nomadproject.io/docs/internals/filesystem#templates-artifacts-and-dispatch-payloads but it would also be nice if we were to validate the jobspec to warn the user against doing this (and perhaps in a later version return an error instead of a warning).

Start with the jobspec validation entry point and the behavior described in issue #7796. Review the documented interaction between volume_mounts and templates, artifacts, and dispatch_payloads, then determine how validation should warn about overlapping destinations; done means the warning covers both host and CSI volumes and has corresponding validation coverage.

Written by the indexing model from the issue text.