Add CUPED variance reduction documentation page

[tbuffington7]

Summary

Adds a new experiments methodology page documenting CUPED (Controlled-experiment Using Pre-Experiment Data), our default variance reduction technique.

The page covers:

• Overview: why variance reduction matters and what CUPED does
• How CUPED works: zero-in-expectation adjustments added to the raw lift
• Default covariates (30-day pre-exposure aggregations + subject properties) and missing-value handling
• How coefficients are selected (variance minimization, not regression adjustment)
• Variance reduction across mean, ratio, and percentile metrics
• FAQ addressing common misconceptions

Also wires the page into the experiments nav and cross-links it from Analysis Methods.

Test plan

• Preview renders correctly (formulas, alert callouts, definition lists, covariate table)
• Page appears in the experiments sidebar between Analysis Methods and Read Experiment Results
• Internal links and the Analysis Methods cross-link resolve

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/tyler.buffington/cuped-documentation/experiments/cuped

Modified Files

• https://docs-staging.datadoghq.com/tyler.buffington/cuped-documentation/experiments/analysis_methods