Add Agent Builder API key creation guidance

[leemthompo]

Summary

• Add a central how-to for creating read-only, management, and short-lived development API keys for Agent Builder.
• Reorganize the permissions page into a concise privilege reference for Kibana features, Elasticsearch access, and spaces.
• Point the Agent Builder API, MCP, A2A, and tutorial pages to the central guidance.

Addresses https://github.com/elastic/docs-content-internal/issues/1417

Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this contribution?

• Yes

Tool and model: OpenAI Codex (GPT-5)

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🔍 Preview links for changed docs

• explore-analyze/ai-features/agent-builder/a2a-server.md
• explore-analyze/ai-features/agent-builder/agent-builder-api-tutorial.md
• explore-analyze/ai-features/agent-builder/api-keys.md
• explore-analyze/ai-features/agent-builder/glossary.md
• explore-analyze/ai-features/agent-builder/kibana-api.md
• explore-analyze/ai-features/agent-builder/limitations-known-issues.md
• explore-analyze/ai-features/agent-builder/mcp-server.md
• explore-analyze/ai-features/agent-builder/permissions.md
• explore-analyze/ai-features/agent-builder/programmatic-access.md

[github-actions]

Elastic Docs Style Checker (Vale)

Summary: 1 suggestion found

💡 Suggestions (1): Optional style improvements. Apply when helpful.

File	Line	Rule	Message
explore-analyze/ai-features/agent-builder/mcp-server.md	76	Elastic.WordChoice	Consider using 'run, start' instead of 'execute', unless the term is in the UI.

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