EX-3105: Added documentation for Experiments Metric Sync CLI + Reorganized Experiments Page

[danyal002]

What does this PR do? What is the motivation?

Adds customer-facing documentation for syncing Datadog experiment metrics from YAML using the Datadog Experiments Metric Sync CLI.

This includes:

• A new “Sync Metrics from YAML” subpage under “Create Experiment Metrics”
• CLI installation and authentication instructions
• A minimal YAML example
• YAML field reference for metric sync definitions
• Recommended plan then execute workflow
• Example GitHub Actions workflow for CI
• Troubleshooting guidance

The motivation is to help customers define experiment metrics in version-controlled YAML, preview changes in pull requests, and apply approved metric changes through CI instead of configuring metrics manually in the Datadog UI.

Merge readiness

• Ready for merge

For Datadog employees:

AI assistance

Used Codex to draft and structure the documentation, based on the Metric Sync CLI behavior and existing Datadog docs patterns. Content was reviewed and adjusted manually.

Additional notes

This PR converts the existing Create Experiment Metrics page into a section by moving it to _index.md, then adds the Metric Sync page as a child page in the docs navigation.

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/danyal.khan/EX-3105/experiments/defining_metrics/metric_sync

Renamed files

• https://docs-staging.datadoghq.com/danyal.khan/EX-3105/experiments/defining_metrics/

[maycmlee]

Created DOCS-15088 for review.