Rebuild the docs around tested examples; shrink README.v2.md to a pitch (#2978)

docs_src/tools/tutorial004.py

@@ -0,0 +1,17 @@
+from pydantic import BaseModel, Field
+
+from mcp.server import MCPServer
+
+mcp = MCPServer("Bookshop")
+
+
+class Book(BaseModel):
+    title: str
+    author: str
+    year: int = Field(ge=1450, description="Year of first publication.")
+
+
+@mcp.tool()
+def add_book(book: Book) -> str:
+    """Add a book to the catalog."""
+    return f"Added {book.title!r} by {book.author} ({book.year})."