docs: revamp landing page with four-pillar card layout (#2531)

Rewrite docs/index.md from a philosophy essay into a landing page
organized around four pillars: Spec-driven by default, Use any coding
agent, Make it your own, and Integrate into your organization.

- Add hero section with GitHub Spec Kit branding and CTA buttons
- Add 2x2 pillar card grid with GitHub Primer color accents
- Add community stats section (96K stars, 200+ contributors, etc.)
- Add navigation cards and footer install CTA
- Move SDD philosophy content to docs/concepts/sdd.md
- Add custom DocFX template overlay with card CSS and dark mode
- Set landing layout for index.md via fileMetadata
- Update toc.yml and docfx.json for new concepts section

docs/concepts/sdd.md

@@ -0,0 +1,46 @@
+# What is Spec-Driven Development?
+
+Spec-Driven Development **flips the script** on traditional software development. For decades, code has been king — specifications were just scaffolding we built and discarded once the "real work" of coding began. Spec-Driven Development changes this: **specifications become executable**, directly generating working implementations rather than just guiding them.
+
+## Core Philosophy
+
+Spec-Driven Development is a structured process that emphasizes:
+
+- **Intent-driven development** where specifications define the "*what*" before the "*how*"
+- **Rich specification creation** using guardrails and organizational principles
+- **Multi-step refinement** rather than one-shot code generation from prompts
+- **Heavy reliance** on advanced AI model capabilities for specification interpretation
+
+## Development Phases
+
+| Phase | Focus | Key Activities |
+|-------|-------|----------------|
+| **0-to-1 Development** ("Greenfield") | Generate from scratch | <ul><li>Start with high-level requirements</li><li>Generate specifications</li><li>Plan implementation steps</li><li>Build production-ready applications</li></ul> |
+| **Creative Exploration** | Parallel implementations | <ul><li>Explore diverse solutions</li><li>Support multiple technology stacks & architectures</li><li>Experiment with UX patterns</li></ul> |
+| **Iterative Enhancement** ("Brownfield") | Brownfield modernization | <ul><li>Add features iteratively</li><li>Modernize legacy systems</li><li>Adapt processes</li></ul> |
+
+## Experimental Goals
+
+Our research and experimentation focus on:
+
+### Technology Independence
+
+- Create applications using diverse technology stacks
+- Validate the hypothesis that Spec-Driven Development is a process not tied to specific technologies, programming languages, or frameworks
+
+### Enterprise Constraints
+
+- Demonstrate mission-critical application development
+- Incorporate organizational constraints (cloud providers, tech stacks, engineering practices)
+- Support enterprise design systems and compliance requirements
+
+### User-Centric Development
+
+- Build applications for different user cohorts and preferences
+- Support various development approaches (from vibe-coding to AI-native development)
+
+### Creative & Iterative Processes
+
+- Validate the concept of parallel implementation exploration
+- Provide robust iterative feature development workflows
+- Extend processes to handle upgrades and modernization tasks