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/source/concepts/apis.md

@@ -1,21 +0,0 @@
-## 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