docs: add package docstring for SDK landing page (#36)

Signed-off-by: Moritz Schmitz von Hülst <mschmitzvonhuelst@gmail.com>

Author: boxcee-interview

crossplane/function/__init__.py

@@ -0,0 +1,63 @@
+# Copyright 2025 The Crossplane Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Python SDK for writing Crossplane composition functions.
+
+This SDK provides the building blocks for implementing [Crossplane composition
+functions](https://docs.crossplane.io/latest/concepts/composition-functions) in
+Python. Composition functions are serverless components that extend Crossplane's
+composition capability, allowing you to programmatically control how infrastructure
+is composed and managed.
+
+## Quick Start
+
+Install the SDK:
+
+```shell
+pip install crossplane-function-sdk-python
+```
+
+Create a composition function by subclassing `Runtime`:
+
+```python
+from crossplane.function.runtime import Runtime
+from crossplane.function.request import RunFunctionRequest
+from crossplane.function.response import RunFunctionResponse
+
+
+class MyFunction(Runtime):
+    def Run(self, request: RunFunctionRequest) -> RunFunctionResponse:
+        # Your composition logic here
+        return response.to(request)
+```
+
+## Modules
+
+- **runtime** — Base class for implementing the function runtime and gRPC server.
+- **request** — Types and utilities for parsing `RunFunctionRequest` messages.
+- **response** — Types and utilities for building `RunFunctionResponse` messages.
+- **resource** — Kubernetes resource types used in function pipelines.
+- **logging** — Structured logging utilities for function output.
+
+## Protobuf Types
+
+The `RunFunctionRequest` and `RunFunctionResponse` types are generated from
+proto3 schema definitions. See the proto modules for API reference:
+
+- [`proto.v1.run_function_pb2`][crossplane.function.proto.v1.run_function_pb2] — Current API
+- [`proto.v1beta1.run_function_pb2`][crossplane.function.proto.v1beta1.run_function_pb2] — Legacy API
+
+Proto-generated fields behave like standard Python types but follow
+[protobuf Python conventions](https://protobuf.dev/reference/python/python-generated/).
+"""