Add lock indicator for docs that are not yet implemented

[Yashb404]

Summary

Docs navigation does not clearly indicate which sections are incomplete; users should see a lock/coming-soon state.

Current Behavior

• Unimplemented docs are not visually distinguished.
• Users can click into placeholders without clear expectation setting.

Expected Behavior

• Missing/incomplete docs are marked with a lock indicator.
• Optional disabled/limited navigation for locked sections.
• Clear tooltip/label such as "Coming soon".

Scope

• Add metadata flag for docs readiness state.
• Render lock icon/tag in sidebar and search results.
• Handle click behavior for locked docs (disabled or informative placeholder).
• Ensure accessibility labels for lock state.

Acceptance Criteria

• Incomplete docs show lock indicator consistently.
• Locked state is visible in nav and searchable lists.
• Interaction with locked docs has clear user feedback.
• Accessibility text explains locked/coming-soon status.