Document migration steps from "task workspace" template to "task" template

[johnstcn]

Document the steps required for template authors to migrate their task templates from using the "AI Prompt" parameter and SidebarAppID to the new format.

Ref:

• feat: add prompt and app_id fields to coder_ai_task terraform-provider-coder#445
• Silently inject task prompt as "AI Prompt" parameter #1079
• chore: tasks: replace exising usage of hard-coded "AI Prompt" parameter with tasks.prompt #979
• Update existing agent CLI modules to reference task.prompt #1065

[johnstcn]

The internal steps we give to template owners to migrate existing templates will be a good input to this.

The overall process will likely look very similar to coder/registry#497