docs: upgrade Docusaurus from v2 to v3

[devcatalin]

Pull request description

Checklist (choose whats happened)

• [ ] breaking change! (describe)
• [ ] tested locally
• [ ] tested on cluster
• [ ] added new dependencies
• [x] updated the docs
• [ ] added a test

Breaking changes

We have to make sure to not use links in format of:

<https://example.com>

and instead do it only this way:

[Title](https://example.com)

Changes

• updated docusaurus and it's dependencies to latest versions

Fixes

• fixed broken mdx files after upgrade

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[rangoo94]

Closing as obsolete, considering that docs have been moved to kubeshop/testkube-docs.