[EX-3085] Document experiment diagnostics

[tbuffington7]

Summary:

• Add an Experiment Diagnostics page with customer-facing warning and resolution guidance.
• Link diagnostics from Experiments overview, Reading Results, Troubleshooting, and the side nav.

Test plan:

• git diff --cached --check
• ruby YAML parse for config/_default/menus/main.en.yaml
• ruby front matter parse for touched Experiments pages

Note: Hugo is not available in this worktree, so I could not run a full local docs build.

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/tbuffington7/EX-3085/experiments/diagnostics

Modified Files

• https://docs-staging.datadoghq.com/tbuffington7/EX-3085/experiments/
• https://docs-staging.datadoghq.com/tbuffington7/EX-3085/experiments/reading_results
• https://docs-staging.datadoghq.com/tbuffington7/EX-3085/experiments/troubleshooting