chore: migrate to hoisted CSF annotations

[edoardolincetto]

Fixes #745

PR Checklist

• My branch is up-to-date with the Upstream master branch.
• The unit tests pass locally with my changes (if applicable).
• I have added tests that prove my fix is effective or that my feature works (if applicable).
• I have added necessary documentation (if appropriate).

Short description of what this resolves:

Storybook 6 introduces hoisted CSF annotations and deprecates the StoryFn.story object-style annotation.

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[dej611]

🎉 Thanks for your contribution!
Looks great, it seems there are few pages still with the warning:

• Le griglie
• Avatar

Can you fix also these?

[sanyamdogra]

Good to merge after the requested changes 👍
Over to @dej611

[edoardolincetto]

Hi @dej611, that's a different issue... 😕
Nothing to fix to remove these warnings. storyFn is never used in the project.

Quoting the Storybook repo:

NOTE: If you're using @storybook/addon-docs, this deprecation warning is triggered by the Docs tab in 6.1. It's safe to ignore and we will be providing a proper fix in a future release. You can track the issue at storybookjs/storybook#13074.

It will be fixed in future releases, so @sanyamdogra I think we can open another specific issue.