[Bug]: Unclear instructions for migrating manager configuration

[yannbf]

Describe the bug

When creating custom themes or configuring Storybook's UI, in Storybook 7 users would have the dependency @storybook/addons and something like this in their manager.js/ts file:

.storybook/manager.ts
import { addons } from '@storybook/addons'
import { create } from '@storybook/theming/create'

const theme = create({
  base: 'light',
  // ...the properties
})

addons.setConfig({
  theme,
  showPanel: true, // show addons panel by default
  panelPosition: 'right', // position addons panel on the right by default
})

In Storybook 7, @storybook/addons was split into @storybook/manager-api and @storybook/preview-api but the migration notes don't really seem clear about what to do in the scenario just described.

When a user upgrades Storybook, they reach the following error:

I'd say we should improve the migration notes, but optimally provide an automigration for this.

[Jkker]

Bump. As of 8.0.0-alpha.13, the addPanel API used in the migration docs is not available in "@storybook/manager-api"

[vanessayuenn]

@Jkker can you link where addons.addPanel() is used? It has been deprecated since 7.x in favour of addons.add() and we should update the docs if it's still there.

[kylegach]

Fwiw, addPanel does not appear anywhere in the 7.6 or 8.0 docs.

[kylegach]

I added a note to MIGRATION.md here: #26039