docs: concepts and building_applications migration (#3534)

# What does this PR do?

- Migrates the remaining documentation sections to the new documentation format

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## Test Plan

- Partial migration

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docs/docs/concepts/index.mdx

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+# Core Concepts
+
+Given Llama Stack's service-oriented philosophy, a few concepts and workflows arise which may not feel completely natural in the LLM landscape, especially if you are coming with a background in other frameworks.
+
+## Documentation Structure
+
+This section covers the fundamental concepts of Llama Stack:
+
+- **[Architecture](./architecture.md)** - Learn about Llama Stack's architectural design and principles
+- **[APIs](./apis/index.mdx)** - Understanding the core APIs and their stability levels
+  - [API Overview](./apis/index.mdx) - Core APIs available in Llama Stack
+  - [API Providers](./apis/api_providers.mdx) - How providers implement APIs
+  - [API Stability Leveling](./apis/api_leveling.mdx) - API stability and versioning
+- **[Distributions](./distributions.md)** - Pre-configured deployment packages
+- **[Resources](./resources.md)** - Understanding Llama Stack resources and their lifecycle
+- **[External Integration](./external.md)** - Integrating with external services and providers
+
+## Getting Started
+
+If you're new to Llama Stack, we recommend starting with:
+
+1. **[Architecture](./architecture.md)** - Understand the overall system design
+2. **[APIs](./apis/index.mdx)** - Learn about the available APIs and their purpose
+3. **[Distributions](./distributions.md)** - Choose a pre-configured setup for your use case
+
+Each concept builds upon the previous ones to give you a comprehensive understanding of how Llama Stack works and how to use it effectively.---
+title: Core Concepts
+description: Understanding Llama Stack's service-oriented philosophy and key concepts
+sidebar_label: Overview
+sidebar_position: 1
+---
+
+# Core Concepts
+
+Given Llama Stack's service-oriented philosophy, a few concepts and workflows arise which may not feel completely natural in the LLM landscape, especially if you are coming with a background in other frameworks.
+
+This section covers the key concepts you need to understand to work effectively with Llama Stack:
+
+- **[Architecture](./architecture)** - Llama Stack's service-oriented design and benefits
+- **[APIs](./apis)** - Available REST APIs and planned capabilities
+- **[API Providers](./api_providers)** - Remote vs inline provider implementations
+- **[Distributions](./distributions)** - Pre-packaged provider configurations
+- **[Resources](./resources)** - Resource federation and registration