Document skill creation from chat and referenced content in skills UI

[leemthompo]

Summary

• Documents creating custom skills conversationally from the chat UI (new H2 section with applies_to for 9.5+)
• Adds Referenced content field to the skill creation stepper with inline applies_to for 9.5+
• Updates Skills API section to no longer imply referenced_content is API-only

Closes https://github.com/elastic/docs-content-internal/issues/1253
Closes https://github.com/elastic/docs-content-internal/issues/1256

Reviewers

• @SiddharthMantri (feat(agent builder): Create skills via chat kibana#265883 — skills via chat)
• @ashatova ([Agent Builder] user-created skills: let user create referenced content kibana#260526 — referenced content in UI)

Test plan

• Verify applies_to annotations render correctly for 9.5+ content
• Confirm referenced content field description matches the UI
• Confirm chat-based skill creation flow matches the shipped feature

🤖 Generated with Claude Code

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🔍 Preview links for changed docs

• explore-analyze/ai-features/agent-builder/custom-skills.md

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Elastic Docs Style Checker (Vale)

Summary: 1 warning found

⚠️ Warnings (1): Fix when the suggestion improves clarity or correctness.

File	Line	Rule	Message
explore-analyze/ai-features/agent-builder/custom-skills.md	100	Elastic.Spelling	'triages' is a possible misspelling.

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