[docs] Restructure documentation navigation and guides for 0.4 - #1244

Linked issue: #1226 (part of #1225)

Purpose of change

The documentation places Agent authoring, model capabilities, built-in agents, and Flink integration at the same level, while its Quickstart entry points differ between the homepage, Overview, and sidebar. This change organizes the existing guidance into a consistent 0.4 hierarchy and updates chapter names and cross-page references.

• Group core authoring under Create an Agent, model-related guides under Calling LLMs, embedding models/vector stores under Vector Search, and sub-agents/pipeline composition under Multi-Agent. Keep provider references beneath their respective model/store guides.
• Align the three Quickstart paths and split the single-Agent walkthrough from the incremental multiple-Agent walkthrough. Move Skills out of Quickstart and incorporate its focused example and complete source links into the Skills guide.
• Give Chat with Models a coherent introduction, request/response example, success/failure handling, multimodal section, and tool-calling flow. Rename Memory Events to Memory Observability and clarify the Agent/ReAct relationship.
• Preserve moved page URLs with aliases and update internal links. No runtime or example source files change.

The language-selection guide (#1227), Resource concepts (#1229), and broader tutorial improvements (#1228) remain separate follow-up work; this change avoids adding empty pages for them.

Tests

• Hugo extended 0.124.1 build passed: 71 pages and 22 aliases. The existing Google Analytics template deprecation warning remains.
• Checked generated internal page/fragment links: zero broken links. All 22 alias targets resolve, and all 14 removed source-page URLs remain available.
• Parsed the six Python snippets in the Chat/Skills guides and checked the linked complete Skills example source paths.
• Checked Apache license headers on changed Markdown files and ran git diff upstream/main --check.
• Inspected local navigation and the text flow diagrams during editing. Runtime suites and live model calls were not run for this documentation-only change.

API

No public API changes.

Documentation

• doc-needed
• doc-not-needed
• doc-included

Was this patch authored or co-authored using generative AI tooling?

• Yes
• No

Generated-by: Codex CLI 0.153.4 (GPT-6)