[dylanlerch]

Context

Feature flags now use rules, phases, and triggers in place of the per-environment targeting form. Projects with existing flags stay on the legacy model until they're upgraded, so the old docs need to stay available.

Changes

• New pages under feature-flags/: overview, getting started, how evaluation works, rules (including evaluation context), phased rollouts, rollout groups, previewing evaluations, managing flags, providers, and upgrading to rules
• Existing pages moved to feature-flags/legacy/ with a banner pointing to the new docs
• feature-flags/targeting and feature-toggles/targeting now redirect to feature-flags/legacy/targeting
• Uses Octopus.FeatureFlags.ClientIdentifier (the old Octopus.FeatureToggles.ClientIdentifier is a deprecated alias)
• Added rollouts and openfeature to the cspell dictionary

How to review

• Pages follow contributing/standards (concept, guide, and reference page types). Unconfirmed facts are marked [VERIFY: …] or [PLACEHOLDER: …], and proposed screenshots are SUGGESTED VISUAL comments. All of these need resolving before this is ready.
• Content is based on the OctopusDeploy source (rule and phase model, trigger resolver, legacy-to-rules converter, and environment editor UI). An SME check on accuracy is still needed.
• The portal's UpgradeToFeatureFlagRules docs link should point to /docs/feature-flags/upgrading-to-rules.
• I haven't built the site locally. Markdown lint, cspell, and a check of internal links and anchors pass.

Resolves BMBB-1256

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