[Dashboards] Add programmatic creation docs and API examples

[florent-leborgne]

Summary

This PR adds documentation for creating Kibana dashboards and visualizations programmatically, as a companion to elastic/kibana#259842 (Visualizations API) and the pending Dashboards API Kibana PR.

New page: explore-analyze/dashboards/create-programmatically.md — covers the Dashboards API, the Lens Visualizations API, the kibana-dashboards agent skill, and the upcoming Agent Builder dashboard tools. All API content is gated at preview 9.4.

Updated pages:

• explore-analyze/dashboards/building.md — adds a pointer to the new programmatic page
• explore-analyze/dashboards/create-dashboard.md — adds a collapsible API dropdown after the agent-skill callout
• explore-analyze/kibana-data-exploration-learning-tutorial.md — adds a forward link in Step 3 and a full API dropdown at the end of the step, so learners can discover the API without skipping the manual tutorial

Notes

• All API examples use applies_to: stack: preview 9.4 / serverless: preview
• API payloads were validated against Kibana main branch
• The Agent Builder tools section references [Epic] Dashboards in chat - Tech Preview kibana#237795 (tech preview, dashboard tools coming in 9.4)
• The kibana-dashboards agent skill is already linked from create-dashboard.md via the {agent-skill} directive; this PR adds a dedicated section in the new overview page

Test plan

• Verify applies_to badges render correctly on all modified pages
• Verify dropdown directives render and collapse correctly
• Verify all links to API reference resolve
• Verify toc.yml addition shows create-programmatically.md in the nav

[github-actions]

🔍 Preview links for changed docs

• explore-analyze/dashboards/building.md
• explore-analyze/dashboards/create-dashboard.md
• explore-analyze/dashboards/create-programmatically.md
• explore-analyze/kibana-data-exploration-learning-tutorial.md

[github-actions]

✅ Vale Linting Results

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[florent-leborgne]

Superseded by #6337. The original draft on this branch was based on the pre-restructure layout of the dashboards programmatic-creation docs and used the old API schema (type: lens, dataset, operation-style x/y). After #5927 split the page into create-dashboards-programmatically.md + create-dashboards-using-ai.md and the Dashboards API tightened its schema, the diff here doesn't apply cleanly. #6337 ports only the still-relevant work — the curl POST in the data-exploration tutorial — onto the new structure, against the verified current schema, with a verification script to keep it honest going forward.