[docs] Create migration guide to new template syntax

Create a migration guide from the original pack syntax to the new version.

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> It isn't clear what does "new Pack template syntax" mean and what migration steps one should do to their packs.
_Originally posted by @slonopotamus in https://github.com/hashicorp/nomad-pack/issues/429#issuecomment-1742719007_

Start by identifying the original pack syntax and the new template syntax in the project, then compare how packs change between them. Document the migration steps and clarify what “new Pack template syntax” means; the guide is done when users can follow it to update an existing pack.

Written by the indexing model from the issue text.