docs: restructure into topical sections and add the four most-asked-for pages (#3044)

docs/get-started/index.md

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+# Get started
+
+New to MCP, or new to this SDK? Start here. These pages take you from nothing to a
+working, tested server: [install the SDK](installation.md), build your
+[first server](first-steps.md), [connect it to a real host](real-host.md), and
+[test it](testing.md) with an in-memory client.
+
+## Run the code
+
+All the code blocks can be copied and used directly: they are complete, working files.
+
+To follow along, paste a block into a `server.py` and open it in the MCP Inspector:
+
+```console
+uv run mcp dev server.py
+```
+
+It is **HIGHLY encouraged** that you write (or copy) the code, edit it, and run it locally. Using it in your own editor is what really shows you the point: how little you write, the autocompletion, the type checks catching mistakes before you run anything.
+
+## You will not be guessing
+
+Every example in these docs is a complete file under [`docs_src/`](https://github.com/modelcontextprotocol/python-sdk/tree/main/docs_src) in the SDK's own repository, and every one of them is exercised by the SDK's test suite through an **in-memory client**:
+
+```python
+import pytest
+from mcp import Client
+
+from server import mcp
+
+
+@pytest.mark.anyio
+async def test_add() -> None:
+    async with Client(mcp) as client:
+        result = await client.call_tool("add", {"a": 1, "b": 2})
+        assert result.structured_content == {"result": 3}
+```
+
+No subprocess, no port, no transport. `Client(mcp)` connects to the server object directly.
+
+If a change to the SDK breaks an example on one of these pages, CI goes red before the page does. The code you read here is the code that runs.
+
+You'll use this yourself in [Testing](testing.md); it's how you test your own servers, too.
+
+## Where to go next
+
+Once you have a server running, the rest of these docs are a reference, not a course.
+Every page stands on its own, so jump straight to what you need:
+
+* What a server exposes (tools, resources, prompts) is **[Servers](../servers/index.md)**.
+* What's available inside the functions you register is **[Inside your handler](../handlers/index.md)**.
+* Getting it in front of clients (stdio, HTTP, your existing FastAPI app) is **[Running your server](../run/index.md)**.
+* Building the other side, an application that *uses* MCP servers, is **[Clients](../client/index.md)**.