docs: create landing page, overview, feature matrix, and roadmap

[moonming]

Parent issue: #240

Summary

Create the top-level docs entrypoint and product-orientation pages for the new documentation set.

Scope

• docs/index.md
• docs/roadmap.md
• docs/overview/what-is-aisix-ai-gateway.md
• docs/overview/deployment-modes.md
• docs/overview/core-concepts.md
• docs/overview/feature-matrix.md

Acceptance Criteria

• The docs homepage routes readers by audience and deployment mode
• AISIX AI Gateway is positioned as the primary product
• AISIX Cloud is positioned as the managed extension
• The feature matrix distinguishes Available, Limited, Preview, and links to the roadmap
• Not-yet-implemented capabilities are captured only in docs/roadmap.md

[coderabbitai]

🔗 Related PRs

#102 - docs: add docs about Introduction, Quick Start, Core Concepts, and Guides [merged]
api7/docs.apiseven.com#212 - docs: 新增 Anthropic 与 OpenAI 协议转换使用指南 [merged]
#8 - docs: update the architecture which is much beautiful [merged]
api7/contents#807 - seo: add 'What Is an AI Gateway' learning center article [merged]
api7/api7.ai#607 - feat(seo): create APISIX vs Istio comparison page [merged]

---

📝 Issue Planner

_{Check the box below or use the @coderabbitai plan command to generate an implementation plan and prompts that you can use with your favorite coding assistant.}

• Create Plan

---

🧪 Issue enrichment is currently in open beta.

You can configure auto-planning by selecting labels in the issue_enrichment configuration.

To disable automatic issue enrichment, add the following to your .coderabbit.yaml:

issue_enrichment:
  auto_enrich:
    enabled: false

💬 Have feedback or questions? Drop into our discord!

[moonming]

Closed by the merge of #249.