docs: refresh custom modes guidance

[Olusammytee]

Summary\n- replace outdated Prompts tab steps with current Modes view guidance\n- clarify YAML-only imports and legacy JSON behavior\n\n## Testing\n- not run (docs-only)\n\nCloses #2147

[changeset-bot]

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[Olusammytee]

Summary: updated custom modes guidance to use the Modes view, clarify YAML-only imports, and explain legacy JSON behavior (closes #2147). Doc-only change, so no changeset required per AGENTS.md. Note: Vercel – docs check is failing on the fork side; likely needs maintainer auth/re-run on Kilo-Org.

[Copilot]

Pull request overview

This PR updates the custom modes documentation to reflect current UI and functionality, directly addressing user confusion reported in issue #2147.

Changes:

• Replaced outdated "Prompts Tab" references with current "Modes View" terminology
• Added clarification that imports only accept YAML files, with guidance for JSON users
• Explained that custom_modes.json is a legacy fallback not kept in sync with custom_modes.yaml