Manager: Add options to hide sidebar and toolbar per story

Closes #22081.

Context

What I did

Added a layoutCustomisations object to the manager layout state, which receives functions to customise whether the toolbar or sidebar should be shown in the UI
Added two getters that get navSize and showToolbar, along with customisations requested by the end user, for use as close to the context of use as possible (within the Preview and Layout components)
Edited documentation to mention the new toolbar function
Made minor type/prop drilling/util generalisation code changes

What's left to do

[ ] Get feedback from core team and @ndelangen
[ ] Add general documentation on layout customisations (left a stub in the relevant place)
[ ] Check how to register the new toolbar.mdx page (if relevant) since toc is gone
[ ] Consider adding an option to hide the panel on certain stories too (e.g. highly curated showcase / example stories where argType editing makes less sense)
[ ] Find a way to test this code as the manager Preview.stories.tsx file is gone
[ ] Discuss the behaviour of the sidebar button in the toolbar, when in fullscreen and the sidebar is hidden
[ ] Discuss the expected behaviour for the sidebar menu in mobile when showSidebar returns false

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

[ ] stories
[ ] unit tests
[ ] integration tests
[ ] end-to-end tests

Manual testing

Edit code/.storybook/manager.tsx with the snippet below
Run yarn storybook:ui
Notice that on docs entries, the toolbar disappears, and it reappears on other entries
Notice that on the unicode docs pages, the sidebar disappears; good luck navigating away from it :D

The snippet:

import { type State, addons } from 'storybook/internal/manager-api';

addons.setConfig({
  layoutCustomisations: {
    showSidebar(state: State, defaultValue: boolean) {
      if (state.viewMode === 'story' && state.storyId.startsWith('😀')) {
        return false;
      }

      return defaultValue;
    },
    showToolbar(state: State, defaultValue: boolean) {
      if (state.viewMode === 'docs') {
        return false;
      }

      return defaultValue;
    },
  },
});

Documentation

[ ] Add or update documentation reflecting your changes
[x] ~If you are deprecating/removing a feature, make sure to update MIGRATION.MD~

Checklist for Maintainers

[ ] When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts
[ ] Make sure this PR contains one of the labels below:

Available labels
- `bug`: Internal changes that fixes incorrect behavior. - `maintenance`: User-facing maintenance tasks. - `dependencies`: Upgrading (sometimes downgrading) dependencies. - `build`: Internal-facing build tooling & test updates. Will not show up in release changelog. - `cleanup`: Minor cleanup style change. Will not show up in release changelog. - `documentation`: Documentation **only** changes. Will not show up in release changelog. - `feature request`: Introducing a new feature. - `BREAKING CHANGE`: Changes that break compatibility in some way with current major version. - `other`: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

_core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>_

Greptile Summary

This PR adds layout customization options to conditionally show/hide the Storybook toolbar and sidebar based on story context, primarily addressing the need to hide toolbars on documentation pages.

Added layoutCustomisations object in manager config with showSidebar and showToolbar functions
Implemented getShowToolbarWithCustomisations and getNavSizeWithCustomisations in layout module to handle visibility logic
Added new documentation in toolbar.mdx explaining conditional toolbar visibility features
Modified Layout and Preview components to integrate customization logic through API props
Added type definitions for layout customization interfaces in addons.ts and api.ts

storybookjs / storybook