Add Manage skills sub-feature to permissions docs

[leemthompo]

Summary

• Adds Manage skills sub-feature to the Kibana privileges section with inline applies_to for 9.5+
• Converts Read and All privilege descriptions from comma-separated sentences to bulleted lists so each new 9.5+ item can carry its own clear applies_to tag

Closes https://github.com/elastic/docs-content-internal/issues/1257

Reviewers

• @machadoum ([Agent Builder] Implement agent builder skills RBAC kibana#256896 — Manage skills sub-feature)

Test plan

• Verify applies_to annotations render correctly on individual list items
• Confirm Manage skills privilege description matches the shipped feature

🤖 Generated with Claude Code

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🔍 Preview links for changed docs

• explore-analyze/ai-features/agent-builder/permissions.md

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