docs: Add Secrets Feature to webUI - Creating Variable Sets

[jfermi]

And add CLI Variable Sets doc link to Concepts and WebUI. For issue - https://github.com/kubeshop/tracetest-cloud/issues/536.

This PR...

Changes

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Fixes

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Checklist

• [ ] tested locally
• [ ] added new dependencies
• [x] updated the docs
• [ ] added a test

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Name	Status	Preview	Comments	Updated (UTC)
tracetest	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 31, 2024 10:38am