Document how to build your own API client

[martin-helmich]

Currently, we have SDKs for JS, Go and PHP (Python possibly incoming).

For users wanting a client in another programming language, we should document how to use common OpenAPI generation tools to build (and maintain) your own client.

Might sit under https://developer.mittwald.de/docs/v2/category/sdks-and-libraries/

Best practices we should mention:

• How to keep your client maintainable (generate your code, as less "hand-crafted" code as possible; do not edit spec files, ...)