chore: API guidelines (#9340)

# Which Problems Are Solved

There were no guideline to how design future APIs and their endpoints.
The V3 documentation was to specific and targeted towards internal
stakeholders.
This PR is intended as base and kept to the minimum. If more details or
additional guideline or rules are needed, they will be added in the
future.

# How the Problems Are Solved

- Removed the V3 description and corresponding examples.
- Provided general guideline for the design of APIs, which includes the
structure, naming, versioning, error handling and more.

# Additional Changes

None

# Additional Context

closes #9184

---------

Co-authored-by: Maximilian <mpa@zitadel.com>
Co-authored-by: Silvan <27845747+adlerhurst@users.noreply.github.com>

CONTRIBUTING.md

@@ -141,6 +141,13 @@ Replace "policeman" with "police officer," "manpower" with "workforce," and "bus
 Ableist language includes words or phrases such as crazy, insane, blind to or blind eye to, cripple, dumb, and others.
 Choose alternative words depending on the context.
 
+### API
+
+ZITADEL follows an API first approach. This means all features can not only be accessed via the UI but also via the API.
+The API is designed to be used by different clients, such as web applications, mobile applications, and other services.
+Therefore, the API is designed to be easy to use, consistent, and reliable.
+Please check out the dedicated [API guidelines](./API_DESIGN.md) page when contributing to the API.
+
 ### Developing ZITADEL with Dev Containers
 
 Follow the instructions provided by your code editor/IDE to initiate the development container. This typically involves opening the "Command Palette" or similar functionality and searching for commands related to "Dev Containers" or "Remote Containers". The quick start guide for VS Code can found [here](https://code.visualstudio.com/docs/devcontainers/containers#_quick-start-open-an-existing-folder-in-a-container)