[agent builder] Add Skills as key concept and document invocation methods

[leemthompo]

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Summary

• Adds Skills as a fourth key concept on the Agent Builder overview page alongside Agent Chat, Agents, and Tools
• Enriches the skills.md intro with a structured breakdown of what a skill bundles (instructions, tools, context) and how skills differ from tools and system prompts
• Documents the three skill invocation methods: automatic discovery, slash commands (with screenshot), and attachment-driven activation
• Updates get-started.md final step to lead with skills as the primary extension point
• Links use case examples to the Security and Observability solution-specific Agent Builder pages

Related

• Tracks against [Internal]: [Docs] Elastic AI Agent: Introduction of Skills and OOTB Skills (9.4) #5719 (core AB collab items)
• Builds on Add Agent Builder skills + dashboards/viz documentation #5868 (skills docs: concept page, custom skills, built-in skills reference, skill creation guidelines)
• Coordinates with Refresh Agent Builder docs for 9.4 UI and navigation #5850 (9.4 UI refresh, which adds skills mentions to the agents overview page)

Test plan

• Verify cross-links resolve: skills.md, builtin-skills-reference.md, custom-skills.md, skill-creation-guidelines.md, solution-specific pages
• Verify skill-slash-command.png renders at correct size
• Confirm no merge conflicts with Refresh Agent Builder docs for 9.4 UI and navigation #5850 on get-started.md

🤖 Generated with Claude Code

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/ai-features/agent-builder/get-started.md
• explore-analyze/ai-features/agent-builder/skills.md
• explore-analyze/ai-features/elastic-agent-builder.md

[github-actions]

Vale Linting Results

Summary: 1 suggestion found

💡 Suggestions (1)

File	Line	Rule	Message
explore-analyze/ai-features/agent-builder/skills.md	25	Elastic.WordChoice	Consider using 'efficient' instead of 'easy', unless the term is in the UI.

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