Add core fabric status guide

[jasnoyaeger]

Summary

• Adds a new product guide explaining how to read, interpret, and troubleshoot core fabric status in VergeOS
• Adds the page to mkdocs.yml nav under System > Nodes

Test plan

• Verify the page renders correctly on the dev server
• Confirm nav placement under Nodes is correct
• Review frontmatter fields (title, description, tags, categories)

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