Update from code changes: document Deep Research API

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Summary

Adds a new Deep Research reference page under the AI guides, covering the durable background investigation API, its lifecycle, effort tiers, permissions, credential handling, and a full start-poll-report example.

Changes

• guides/ai-agents/deep-research.mdx — new page describing what Deep Research is, when to use it, the four effort tiers and their budgets, permissions (create:AiDeepResearch scope, Developer/Admin only), credential lifecycle, all four API endpoints (start, read, events with cursor pagination, cancel), and a bash example.
• guides/ai-overview.mdx — mention Deep Research in the capabilities-at-a-glance and "if you want to…" tables.
• docs.json — add the new page to the AI agents nav group.

Context

Reflects the new Deep Research API from these upstream PRs:

• feat(deep-research): run managed investigations lightdash#25514 — feat(deep-research): run managed investigations
• feat(ai): expose the Deep Research API lightdash#25524 — feat(ai): expose the Deep Research API
• feat(deep-research): increase effort token budgets lightdash#25530 — feat(deep-research): increase effort token budgets
• feat(deep-research): gate runs with a dedicated scope lightdash#25539 — feat(deep-research): gate runs with a dedicated scope

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