chore: documentation (#76)

* chore: update docstring example to use python code block

* docs: add documentation

* feat: add docs build + fix other workdlows

* fix: add missing return type

docs/module/exceptions.md

@@ -0,0 +1,82 @@
+# Exceptions
+
+Structured API exceptions with consistent error responses and automatic OpenAPI documentation.
+
+## Overview
+
+The `exceptions` module provides a set of pre-built HTTP exceptions and a FastAPI exception handler that formats all errors — including validation errors — into a uniform [`ErrorResponse`](../reference/schemas.md#fastapi_toolsets.schemas.ErrorResponse) shape.
+
+## Setup
+
+Register the exception handlers on your FastAPI app at startup:
+
+```python
+from fastapi import FastAPI
+from fastapi_toolsets.exceptions import init_exceptions_handlers
+
+app = FastAPI()
+init_exceptions_handlers(app)
+```
+
+This registers handlers for:
+
+- [`ApiException`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ApiException) — all custom exceptions below
+- `RequestValidationError` — Pydantic request validation (422)
+- `ResponseValidationError` — Pydantic response validation (422)
+- `Exception` — unhandled errors (500)
+
+## Built-in exceptions
+
+| Exception | Status | Default message |
+|-----------|--------|-----------------|
+| [`UnauthorizedError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.UnauthorizedError) | 401 | Unauthorized |
+| [`ForbiddenError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ForbiddenError) | 403 | Forbidden |
+| [`NotFoundError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NotFoundError) | 404 | Not found |
+| [`ConflictError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ConflictError) | 409 | Conflict |
+| [`NoSearchableFieldsError`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.NoSearchableFieldsError) | 400 | No searchable fields |
+
+```python
+from fastapi_toolsets.exceptions import NotFoundError, ForbiddenError
+
+@router.get("/users/{id}")
+async def get_user(id: int, session: AsyncSession = Depends(get_db)):
+    user = await UserCrud.first(session, filters=[User.id == id])
+    if not user:
+        raise NotFoundError
+    return user
+```
+
+## Custom exceptions
+
+Subclass [`ApiException`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.ApiException) and define an `api_error` class variable:
+
+```python
+from fastapi_toolsets.exceptions import ApiException
+from fastapi_toolsets.schemas import ApiError
+
+class PaymentRequiredError(ApiException):
+    api_error = ApiError(
+        code=402,
+        msg="Payment required",
+        desc="Your subscription has expired.",
+        err_code="PAYMENT_REQUIRED",
+    )
+```
+
+## OpenAPI response documentation
+
+Use [`generate_error_responses`](../reference/exceptions.md#fastapi_toolsets.exceptions.exceptions.generate_error_responses) to add error schemas to your endpoint's OpenAPI spec:
+
+```python
+from fastapi_toolsets.exceptions import generate_error_responses, NotFoundError, ForbiddenError
+
+@router.get(
+    "/users/{id}",
+    responses=generate_error_responses(NotFoundError, ForbiddenError),
+)
+async def get_user(...): ...
+```
+
+---
+
+[:material-api: API Reference](../reference/exceptions.md)