[Nightshift] Add docs for the Nightshift UI

[mdbirnstiehl]

Summary

This PR closes #7372 and #7371 and creates the docs for the Investigations, Memory, and UI aspects of Nightshift. These files are currently hidden as they are part of a private preview.

The docs cover:

Nightshift UI: The landing page, Significant Events list, event flyout, and chat
Investigations: How autonomous investigations work, what they produce, and how to view them
Memory: How the persistent knowledge base is built, the workflows that maintain it, and how agents use it during investigations

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes
• No

Tool(s) and model(s) used: Claude Code Sonnet 4.6

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Elastic Docs Style Checker (Vale)

Summary: 1 warning found

⚠️ Warnings (1): Fix when the suggestion improves clarity or correctness.

File	Line	Rule	Message
solutions/observability/nightshift/memory.md	76	Elastic.DontUse	Don't use 'just'.

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The Vale linter checks documentation changes against the Elastic Docs style guide. To use Vale locally or report issues, refer to Elastic style guide for Vale.

[github-actions]

🔍 Preview links for changed docs

• solutions/observability/nightshift/investigations.md
• solutions/observability/nightshift/memory.md
• solutions/observability/nightshift/nightshift.md
• solutions/observability/streams/significant-events/index.md

[roshan-elastic]

Hey @mdbirnstiehl, these look great.

Do we think we could add a call-out one pages to ask people to leave feedback via our feedback mechanism?

This will go straight to our Slack channels and will be a good way to start collecting feedback.

[mdbirnstiehl]

Hey @mdbirnstiehl, these look great.

Do we think we could add a call-out one pages to ask people to leave feedback via our feedback mechanism?

This will go straight to our Slack channels and will be a good way to start collecting feedback.

Sure thing @roshan-elastic! Added in abe06d3