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/apis/index.mdx

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+---
+title: APIs
+description: Available REST APIs and planned capabilities in Llama Stack
+sidebar_label: APIs
+sidebar_position: 1
+---
+
+# APIs
+
+A Llama Stack API is described as a collection of REST endpoints. We currently support the following APIs:
+
+- **Inference**: run inference with a LLM
+- **Safety**: apply safety policies to the output at a Systems (not only model) level
+- **Agents**: run multi-step agentic workflows with LLMs with tool usage, memory (RAG), etc.
+- **DatasetIO**: interface with datasets and data loaders
+- **Scoring**: evaluate outputs of the system
+- **Eval**: generate outputs (via Inference or Agents) and perform scoring
+- **VectorIO**: perform operations on vector stores, such as adding documents, searching, and deleting documents
+- **Telemetry**: collect telemetry data from the system
+- **Post Training**: fine-tune a model
+- **Tool Runtime**: interact with various tools and protocols
+- **Responses**: generate responses from an LLM using this OpenAI compatible API.
+
+We are working on adding a few more APIs to complete the application lifecycle. These will include:
+- **Batch Inference**: run inference on a dataset of inputs
+- **Batch Agents**: run agents on a dataset of inputs
+- **Synthetic Data Generation**: generate synthetic data for model development
+- **Batches**: OpenAI-compatible batch management for inference