Review and update migration from workflowai docs

[anyacherniss]

Reference with more specific feedback/todos: #318

File name: private.migrate-from-workflowai.mdx

High level description:

Step 1: some of the content on the existing migration page focuses heavily on the proxy endpoint. The request is to remove that content/adjust other content that references features/formatting exclusive to the proxy endpoint because so few users were actually using this feature, it could cause confusion.

Step 2: include copy/pastable prompts that a user can use to ask claude/cursor to do the migration for them

@guillaq I can probably do most of this update and all of step 2. This todo is mostly for us to sync so I can make sure I understand the changes that need to be made (or if easier for you to make the changes directly to the docs that's all good too)