Enhance experiment designing documentation

[jaredlockhart]

Warning

This PR is based on #779 (Rollouts enhancement), which is based on #777 (Firefox Labs docs). Review those PRs first — this one only adds the experiments article changes on top.

Summary

Expands the experiment designing article from ~25 lines to a comprehensive guide, drawing on the experiment brief template to walk users through the full design process. File renamed from designing.md to experiments.md to match its sidebar label.

Changes

Restructured into clear phases:

• When to use / when not to use — with cross-links to rollouts and Firefox Labs
• How experiments work — lifecycle and key concepts (branches, enrollment, observation, sizing)
• Designing your experiment — 7-step process from hypothesis through office hours
• Build and de-risk — feature enablement, exposure events, QA, risk review
• Launch, monitor, learn — sizing, launch, monitoring, and recording results/next steps

New content from the experiment brief template:

• Expanded hypothesis format with the "If we / for / then we will see / because" template
• Success metrics section (primary, guardrail, feature-specific, segments)
• Branch design guidance (control vs treatments, what to document for each)
• Targeting considerations table (application, version, locales, advanced targeting, exposure conditions, exclusions)
• Build and de-risk checklist (Nimbus API, exposure events, QA, risk review)
• Results decision framework (ship it, kill it, iterate) with link to promote-to-rollout
• Note about message consult requirement for messaging experiments

Housekeeping:

• File renamed: designing.md → experiments.md
• Frontmatter id/slug updated: designing → experiments
• Sidebar and cross-link references updated
• Workflow diagram moved to end
• Removed Jetstream name reference
• Corrected observation period to default 3 weeks (4 weeks total)

fixes #780

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