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>

docs/sidebars.js

@@ -737,10 +737,6 @@ module.exports = {
           type: "category",
           label: "V3 (Preview)",
           collapsed: false,
-          link: {
-            type: "doc",
-            id: "apis/v3",
-          },
           items: [
             {
               type: "category",