docs(holdout): add Holdouts product documentation

[chris-absmartly]

Add a Holdouts section under Product Documentation covering the new holdout groups feature:

• overview: what a holdout measures, one holdout per tracking unit, the Holdout vs No Holdout comparison, traffic allocation, automatic experiment inclusion, periods and resets
• setting-up-a-holdout: walkthrough of the Configure Holdout screen
• reading-holdout-results: the Overview and Iterations tabs

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

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Run ID: 35633c6e-570b-441a-ba24-e6aa13f7c289

📥 Commits

Reviewing files that changed from the base of the PR and between 5f77e19 and 845d9f2.

⛔ Files ignored due to path filters (4)

• static/img/holdout/configure-holdout.png is excluded by !**/*.png
• static/img/holdout/holdout-iterations.png is excluded by !**/*.png
• static/img/holdout/holdout-results.png is excluded by !**/*.png
• static/img/holdout/holdouts-list.png is excluded by !**/*.png

📒 Files selected for processing (4)

• docs/web-console-docs/holdout/_category_.json
• docs/web-console-docs/holdout/overview.mdx
• docs/web-console-docs/holdout/reading-holdout-results.mdx
• docs/web-console-docs/holdout/setting-up-a-holdout.mdx

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch holdout

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