Documentation: Add addon-themes docs to the sidebar

[yannbf]

Closes #25042

What I did

The sidebar entry for addon-themes was disabled, and therefore its documentation was not available

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

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

Documentation

• Add or update documentation reflecting your changes
• 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/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>

[jonniebigodes]

@yannbf, thanks for taking the time to put this together and helping us improve the documentation. Appreciate it 🙏 ! However, before this can be merged, there are a couple of things that need to happen.
The addon needs to be added to the essentials package, or at least made available with every Storybook installation, so that it can be considered an essential addon that developers could use, or make it a part of their daily workflow to make their lives easier. We did something similar at various stages with the interaction testing addon, the measure and outline addons and always referenced it as such in the documentation.
Additionally, on the documentation front, there's still some work to be done as missing information regarding APIs and other useful bits of information that could go into the documentation as well so that they can help our users.

Until we have a decision on this, I'm adding the ci do not merge label to avoid having it accidentally merged and published into the documentation.

Let me know and we'll take it from there.

[github-actions]

	Fails
🚫	PR is marked with "ci: do not merge" label.

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[vanessayuenn]

@jonniebigodes we won't be able to add addon-theme to essentials before 8.0 as it currently does not have an official codeowner and the team is rather swamped with the upcoming release. However, given how widely used the addon is, it will be very beneficial to have its documentation more accessible from the side bar. If not under essential addons, can you recommend a more appropriate place to put it?

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