DOC-57 Add mermaid config and fix diagrams in platform section

[aimeeu]

Fixes DOC-57

• Add mermaid config so diagrams in platform section render.
• Update docusaurus.config.js to match v3.x format.
• Fix broken images that I found when looking at the mermaid diagrams

https://deploy-preview-110--vcluster-docs-site.netlify.app/docs/

Diagram previews: https://deploy-preview-110--vcluster-docs-site.netlify.app/docs/platform/get-started/explore#inactivity-detection https://deploy-preview-110--vcluster-docs-site.netlify.app/docs/platform/virtual-clusters/sleep-mode#inactivity-detection

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