Update Docusaurus install instructions in readme

[misterpantz]

Description

Added a little more info to the Docusaurus 🦖 install step.

Related issue

none

Types of changes

• Functional documentation bug fix (i.e., broken link or some other busted behavior)
• New functional doc capabilities (i.e., filter search results)
• New content
• Revision to existing content
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist

• I have read the README document about contributing to this repo.
• I have tested my changes locally and there are no issues.
• All commits are signed.

[github-actions]

Starting creation of the PREview environment...

[github-actions]

PREview is ready at https://mondoo-pre-docs-tinyfix.storage.googleapis.com/docs/index.html