[EXP-GA] Move Experimentation docs to top-level navigation

[iadjivon]

What does this PR do? What is the motivation?

Fixes DOCS-13628

SCOPE

• Moves Experimentation documentation from /product_analytics/experimentation/ to /experiments/
• Renames "Experimentation" to "Experiments" across docs
• Adds aliases to redirect old URLs
• Updates internal links and further_reading references
• Updates side nav to place Experiments under Digital Experience as a top-level item
• Adds experiment-wui icon to the nav entry

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OUT OF SCOPE

• Quick legacy content edits
• I have cards for reviewing these pages in early April when the new UI is pushed

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Merge instructions

Merge readiness:

• Ready for merge

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Additional notes

Pages moved:

• _index.md (Planning and Launching Experiments)
• defining_metrics.md
• minimum_detectable_effect.md
• reading_results.md
• troubleshooting.md

Pages renamed:

• Rename content/en/{experimentation => experiments}/_index.md
• Rename content/en/{experimentation => experiments}/defining_metrics.md
• Rename content/en/{experimentation => experiments}/minimum_detectable_effect.md
• Rename content/en/{experimentation => experiments}/reading_results.md
• Rename content/en/{experimentation => experiments}/troubleshooting.md

[github-actions]

Preview links (active after the build_preview check completes)

Renamed files

• https://docs-staging.datadoghq.com/ida.adjivon/DOCS-13628-experiments-top-level-nav/experiments/
• https://docs-staging.datadoghq.com/ida.adjivon/DOCS-13628-experiments-top-level-nav/experiments/defining_metrics
• https://docs-staging.datadoghq.com/ida.adjivon/DOCS-13628-experiments-top-level-nav/experiments/minimum_detectable_effect
• https://docs-staging.datadoghq.com/ida.adjivon/DOCS-13628-experiments-top-level-nav/experiments/reading_results
• https://docs-staging.datadoghq.com/ida.adjivon/DOCS-13628-experiments-top-level-nav/experiments/troubleshooting