docs: Add AsyncSession and async tools tutorial (#626)

docs/en/docs/tutorial/async-session/index.md

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+# Async Tools
+
+SQLModel supports asynchronous execution via SQLAlchemy's `ext.asyncio`. This is very useful when working with high-concurrency applications, such as FastAPI.
+
+## `AsyncSession` and `create_async_engine`
+
+Instead of `create_engine`, you use `create_async_engine`. And instead of `Session`, you use `AsyncSession`.
+
+```python
+from sqlmodel import SQLModel, Field, select
+from sqlmodel.ext.asyncio.session import AsyncSession
+from sqlalchemy.ext.asyncio import create_async_engine
+
+class Hero(SQLModel, table=True):
+    id: int | None = Field(default=None, primary_key=True)
+    name: str = Field(index=True)
+    secret_name: str
+    age: int | None = Field(default=None, index=True)
+
+# Note the +asyncpg in the database URL
+sqlite_url = "postgresql+asyncpg://user:password@localhost/db"
+engine = create_async_engine(sqlite_url, echo=True)
+
+async def create_db_and_tables():
+    async with engine.begin() as conn:
+        await conn.run_sync(SQLModel.metadata.create_all)
+
+async def create_heroes():
+    hero_1 = Hero(name="Deadpond", secret_name="Dive Wilson")
+    async with AsyncSession(engine) as session:
+        session.add(hero_1)
+        await session.commit()
+        await session.refresh(hero_1)
+        print("Created hero:", hero_1)
+
+async def read_heroes():
+    async with AsyncSession(engine) as session:
+        statement = select(Hero).where(Hero.name == "Deadpond")
+        results = await session.exec(statement)
+        hero = results.first()
+        print("Hero:", hero)
+```
+
+## Using with FastAPI
+
+When integrating with FastAPI, you can use `AsyncSession` as a dependency.
+
+```python
+from fastapi import FastAPI, Depends
+from sqlmodel.ext.asyncio.session import AsyncSession
+
+app = FastAPI()
+
+async def get_session():
+    async with AsyncSession(engine) as session:
+        yield session
+
+@app.get("/heroes/")
+async def read_heroes(session: AsyncSession = Depends(get_session)):
+    statement = select(Hero)
+    result = await session.exec(statement)
+    heroes = result.all()
+    return heroes
+```