API Prefix vs Endpoint path

# The problem
currently, the default values for the API prefix and the end point paths are wrong.

```
ApiClient::api_prefix = wp-json/wp/v2
PostsRepository::path = posts
Final path = wp-json/wp/v2/posts
```
The current repositories work fine because they are all part of the `wp/v2` end points. However, when creating custom repositories not under the `wp/v2` path, you would not be able to properly set the path from the custom repository.

# Example
An example of this would be a scenario where you would want to create a new repository that interacts with the [WP API Menus plugin](https://wordpress.org/plugins/wp-api-menus/). Their endpoints are `wp-json/wp-api-menus/v2/`.

# The Fix
`ApiClient::api_prefix` should be set to `wp-json` and the current repositories path should be `wp/v2/{name}`

Read ApiClient and the current repository definitions, especially PostsRepository, to trace how the API prefix and endpoint paths are combined. Update the defaults so the final path remains wp-json/wp/v2/posts while custom repositories can target paths such as wp-json/wp-api-menus/v2; verify both existing and custom repository URLs.

Written by the indexing model from the issue text.