Enable _clear_state and ensure test independence

[maxschulz-COL]

Description

• implement the private _clear_state function as suggested by @ned2
• apply to all tests to ensure test independence
• adjust preliminary warnings that guide towards this solution if cells are re-evaluated in Jupyter
• Replace tech_logos.png

Screenshot

Checklist

• I have not referenced individuals, products or companies in any commits, directly or indirectly
• I have not added data or restricted code in any commits, directly or indirectly
• I have updated the docstring of any public function/class/model changed
• I have added the PR number to the change description in the changelog fragment, e.g. Enable feature XXX ([#1](https://github.com/mckinsey/vizro/pull/1)) (if applicable)
• I have added tests to cover my changes (if applicable)

Types of changes

• Docs/refactoring (non-breaking change which improves codebase)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Notice

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[huong-li-nguyen]

Thanks you two for fixing @maxschulz-COL @ned2! 🚀

@maxschulz-COL - could you add @ned2 to our list of contributors in our authors.md file to give him credits for the proposed solution?

[huong-li-nguyen]

Thanks for cleaning up! 🧹 🔥

[antonymilne]

Thank you for the fix @maxschulz-COL and @ned9!

I just have a couple of small comments and one concern about the test fixture but otherwise looks great 💯

In the near future I think we might want to move some of this resetting behaviour so it always runs automatically in our Vizro.__init__ or similar, and also make the rest that's left in Vizro._reset into public API. This can be handled as part of #111 because it's all related.

[petar-qb]

It looks good!